Keenetic Linear: omnivorous SIP gateway to which a disk phone can be connected

The demand for IP telephony among small businesses is quite high: in situations when mobile communication simply does not work (on the ground floors or in basements), or when a company needs to consolidate a telephone communication channel and wrap voice traffic in CRM with analytics, nothing has been invented better than a SIP channel yet. In addition, it is now possible to rent phone numbers from other regions quite cheaply, providing work anywhere in the country without having to be physically present in the region and open your office. For such cases, as a rule, cheap SIP phones or VOIP gateways are used, and here is a similar solution proposed by Keenetic, whose lineup is slowly but surely growing into the SmB/SOHO segment.

^^^Video with sound^^^

The ideology here is simple: Keenetic is the most affordable and most convenient tool for creating a point of presence on Edge objects. Whether you're opening a pick-up point, barbershop, or convenience store, all you need is a cable from a provider with internet access to get a modern network that can be serviced remotely via the cloud. With the growth of requirements, it can be upgraded to Mesh or Wi-Fi with roaming, create VPN connections, set up a backup channel via LTE, install an IP camera with recording on a USB drive ... and much more. The most important thing is that for almost any task you do not need a top-end expensive router from the Ultra or Giga series: the software for Keenetic is universal, and budget models will be quite suitable for many objects.

The Keenetic Linear VoIP gateway is positioned as a Plug'n'play solution for your Keenetic router, which already contains data from almost all VoIP providers, which just needs to be connected to the Keenetic router, enter the login and password of the SIP service there and connect the phone. Everything about everything will take no more than an hour if you see Keenetic Linear for the first time in your life, and five minutes if you configure it for the second or third time. As usual, everything can be configured over the Internet using the KeenDNS cloud service. The Keenetic company's press brief says that the Linear model will work with any analog phone: even a DECT, even an old disk, and we decided to test the last statement on our own experience by connecting a 30-year-old telephone via VoIP.

Who needs a disk phone in 2022 anyway?

I was surprised when I found out that the demand for disk telephones is still stable. They are bought for the equipment of some themed hotels, conducting quests and creating a retro entourage.

Quest "phobia"

Among home users, a disk phone is the easiest way to connect a grandfather/ grandmother in those locations where there is already an Internet connection, but there is no telephone line, or there is, but it is expensive. Now you can buy these devices used, and a good quality phone will cost 10-15$, plus you will need either an adapter from an old outlet to RJ11, or just a compressed telephone cable that needs to be soldered to the device.

What you need to know before buying a disk phone for VoIP

Radio tubes, DECT devices and office digital-analog phones do not have any features and compatibility problems, but when working with a disk device, there may be some difficulties that you need to know in advance. Firstly, as we remember, telephone lines of the past transmitted not only a signal, but also power for the device, and its voltage was up to 120 V. When calling on the phone, a powerful electromagnet is triggered, which sets in motion a hammer knocking on the bell, producing a call sound, and at this moment many VoIP gateways designed for office or DECT phones with separate power stumble. Even looking at Keenetic Linear, at this small USB-powered box, it's hard to imagine that powered by a router, it will pull a call to a telephone exchange. In addition, there are two RJ11 sockets here, which means that twice as much power is needed.

I recommend that you, when choosing an entourage disk phone, still abandon Soviet models, and choose Polish, Bulgarian or, if you're lucky, German. Like modern smartphones, they all "look the same", but the difference in circuitry is significant.

But even in extreme cases, if your phone is too demanding of power, the maximum that incompatibility threatens is that it will not ring when called, or it will make a sound too quiet. Dialing, volume and quality of the conversation will not be affected.

Keenetic Linear has two RJ11 sockets, into which you can connect several devices in parallel, but in this case it is better to consider phones with volume control, or those that ring a ringtone through the speaker, and do not ring a bell, and these are almost all push-button wired retro phones.

How to connect and configure Keenetic Linear?

Keenetic Linear is not a separate product, but an addition to Keenetic routers that have a USB port. As it is fashionable to say now, this is a "software and hardware complex" with a serial USB interface. That is, you cannot connect it to a computer or laptop: you definitely need Keenetic with KeenOS firmware version higher than 3.7.5, having Internet access. In our test, we will use an old simple Viva (KN-1910) to show that it is not necessary to choose the most sophisticated model for VoIP.

The IP telephony service will work even when Keenetic is switched to repeater or adapter mode, so if your organization uses a Mesh network, you can connect your phone to the device that is more convenient for you.

The first step is to log into the router's web interface and install the "Telephone Exchange" components. At the same time, the device will automatically update the firmware and reboot, after which you can connect Keenetic Linear to any of the USB ports of the router (it does not matter whether USB 2.0 or USB 3.x).

Now in the main menu you will have the item "telephone exchange", which will have all the settings related to VoIP. Let's start with the last one - the call log, which can be stored on both internal and external media, but there are no call records themselves. To export to the analytics system, you can upload the log in .csv format, but only manually.

SIP communication is configured in the "Telephone line" section. The minimum that needs to be done to make the telephone connection work is to add a line. In the window that opens, select one of the 24 pre-installed providers (for example Deutche Telekom). If your provider is present in the list, you only need to enter the SIP number and password provided in your personal account. If your provider is not in this list, then select "other" and manually enter the addresses of the SIP servers.

In our test, we used a service for which the UDP, TCP and TLS transport protocols were available. When using UDP, there were problems with the audibility of the subscriber, so I recommend abandoning this protocol. There were no problems with the TCP protocol at all, but in order to work through an encrypted TLS connection, we had to disable the mandatory authentication of the server through certification authorities.

If you use several SIP providers, for example, for phone numbers of different cities, you can add additional lines and set the priority from which line to make calls by default, and if necessary, add a prefix to the dialed numbers.

Keenetic Linear supports only two types of codecs: G.711A and G.711U, which is implemented in almost all SIP equipment. There is no zoo of codecs here, and the current capabilities are sufficient when working with a stream of 64 Kbit/ s, that is, it will work both by wire and via LTE.

There is no built-in echo cancellation system in Keenetic Linear, although the chip on which the adapter is built has such a function. In our test, the echo was manifested when calling the mobile numbers of the Tele2 operator, and changing the direction in the interface of the VoIP provider service did not help. Guided by the Asterisk knowledge base, I came to the conclusion that the echo appears on the side of the disk phone, and frankly, I did not dare to demand an anechoic operation in digital networks of the 21st century from a device older than me. There were no such problems with the DECT tube.

I contacted the developers of Keenetic Linear, and they suggested that for those of our readers who encounter echo problems, set the impedance 900, 1200 and 900_INF_2160 on the FXS ports with the commands described below. You need to enter them through the command line interface by connecting to the router via SSH:

1) impedance 900:

nvox fxs port 1 impedance 900_0_0
nvox fxs port 2 impedance 900_0_0 

2) impedance 1200:

nvox fxs port 1 impedance 1200_0_0
 nvox fxs port 2 impedance 1200_0_0

3) impedance 900_INF_2160:

nvox fxs port 1 impedance 900_INF_2160 
nvox fxs port 2 impedance 900_INF_2160

Example of command output:

(config)> nvox fxs port 1 impedance 900_0_0
Nvox::Manager: Set FXS port 1 Impedance preset to "900_0_0".

You can check the current port impedance values with the command “show nvox fxs-ports”.

Example of command output:

(config)> show nvox fxs-ports
id: 1 
name: phone 1
int-number: 1 
status: enabled 
volume-rx: -5
volume-tx: -5
impedance: 600_0_0 
hash-ends-dial: enabled
echo-cancellation: enabled

id: 2
name: phone 2
int-number: 2
status: enabled
volume-rx: -7
volume-tx: -7
impedance: 220_820_115
hash-ends-dial: enabled
echo-cancellation: enabled

After that, the echo problems should go away.

Features of using a disk phone

Those who have come across IP telephony are used to the fact that a well-established service works "like clockwork", and today it is even more comfortable type of communication than simple city lines. Keenetic Linear fits completely into this concept, without requiring any advanced settings from the administrator, and just works, as they say, "out of the box". However, the disk phone of the last century has its own peculiarities.

Is it possible to dial 11-digit mobile numbers?

Yes, cellular subscriber numbers must be dialed through the "eight". Of course, when calling a dozen subscribers, the index finger will cry from the tension of the disk rotation, but there are no technical problems in this regard. Using the dialing rules, you can set up speed dial to reduce the number of digits dialed during outgoing calls. For example, by dialing 01, the call will go to the number +14951234567, and when dialing 02, it will go to the number +19037654321.

^^^ Please turn on sound ^^^

In Keenetic Linear, a pause between pulses is configured, but you will have to change these settings only if your phone has an old, unoiled mechanism, because of which the disk rotates too slowly, or vice versa too fast.

What are the features of receiving calls?

We talked about the voltage and the volume of calls above, but in addition there are other tricks. For example, you should pick up the phone after the second call, because if you do it earlier, the connection may not be established. It will not be possible to reset the call by briefly lifting the handset, as old bureaucrats did - the call will continue to come, and the phone will ring. Attention should be paid to this point when using Keenetic Linear in theatrical productions, filming and art installations.

How to dial "extension" numbers through an asterisk or a grid?

No way. Extension numbers, as well as the telephone menu, are the lot of push-button devices with tone dialing. Parallel call and call hold modes are also supported for them.

Is it possible to connect two Linear to one router?

At the moment - no, but the manufacturer has such plans.


Perhaps in your business VoIP phone will be used on a case-by-case basis: during exhibitions or during the holiday season, and you will not want to spend extra money on a SIP gateway. If you built a computer network based on Keenetic in advance, consider yourself lucky, because conditionally for two Internet subscription fees you will receive a gateway for 2 FXS ports with a simple and convenient interface that can be configured remotely via the KeenDNS cloud. For a home, this is generally an ideal option if you need to install a stationary telephone for an elderly person.

For everyday business use, I would look at Keenetic Linear in terms of integration needs: do you need access to the VoIP gateway via API or command line? Do you need to download the call log from the device and integrate it into the analytics system? Do you need any other codecs and advanced settings? For all its convenience, Keenetic Linear is deprived of Enterprise functions, and it is unlikely that a potential audience will require them.

Michael Degtjarev (aka LIKE OFF)

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