Connecting Asterisk to analog line

Hello, not sure this should be in general or hardware. I have never set up an Asterisk switch before but I have the server installed and running and I have some general questions.

Now my interest is in vintage telephones and using them with modern networks. I have a 4G modem with an analog phone socket. So plugging a phone into that means one can dial the number of the modems sim car and the phone will ring. And vice versa you can ring out.

Can I treat this modem for all intents and purposes as an ATA? It seems to be exactly what it does. If not, what am I missing?

And then what I need on the server is an FXO port to connect Asterisk to the phone line, and one or multiple FXS ports to connect any analog phones to the asterisk server.

I may have gotten the FXS/FXO terminology confused…

Normally ATA refers to a SIP to analogue adaptor. What you are describing is a 4G mobile air interface to analogue adaptor. Standard commercial hardware for Asterisk does not implement the network side of the mobile interface, and hardware that does will generally require a radio telephony licence, which is likely not to be available in many countries. I can’t see how this helps you with Asterisk.

Especially if these phones are pulse dialing. you may need the, expensive,. Sangoma PCI cards, rather than an ATA, as you may need more control over the configuration than typical ATAs provide.

My understanding was the FXS ports purpose was basically to connect to the outside world of an analog phone system. And that’s what te 4G modems port is already simulating. So I should be able to dial out using asterisk and the 4G modem? Or not?

The way I see it, Asterisk does not need to care about the mobile side or radio stuff, it should be able to treat the 4G modem as a wall socket for a Plain Old Telephone Service. The 4G modem with it’s sim card handles all of the other routing.

Mainly that’s what I want Asterisk for, to work with an FXS/FXO card and dial out.

An FXS port is used to connect to an analogue station, which in practice means a phone. An FXO port it to connect to an analogue office, where office is the term used for an exchange.

Strictly speaking, an ATA only offers FXS ports, although, as the hardware is similar, devices are sold which do both. One that offers FXO ports would normally be called a gateway.

To connect Asterisk, through the 4G devices FXS ports, you need FXO ports on Asterisk, Most people, particularly small users, do that using a SIP to analogue gateway, connected, over Ethernet, to the machine running Asterisk. However you can buy PCI and PCIe cards that plug into the machine’s motherboard.

To use your analogue phones, with Asterisk, you will need a SIP ATA device, or a PCI/PCIe card with FXS modules. The SIP ATA may also have gateway capabilities.

If the phones are not modern, touch tone phones, your choice of ATA may be limited.

Using a 4G modem back to back with an analogue interface on Asterisk, is likely to lose you supervision signals, like answered and cleared, which are handled by mobile networks, but not well handled over analogue lines, so, you should be looking for a 4G gateway that interfaces using SIP, not using analogue as an intermediary.

There was third party support for ones that interfaced over USB, but those are now, effectively, unsupported.

Ok so the setup would look something like:

Analog phone <> FXS Port (pci card or ata) / Asterisk server <> 4G gateway / modem

Only connect the analog phones to asterisk and all outgoing traffic into the world is done via say VoLTE and I forget about using the phone port on the modem.

Can the 4G modem itself act as a 4G gateway? Depends on the modem?

That assumes that the 4G Gateway is SIP based. If, as originally described, it was analogue based:
Server running Asterisk daemon <> FXO port (PCI or SIP gateway) <> analogue to 4G gateway. (“modem”)

It’s a TP-Link MR-6500V with an analog out for old style phones. But it supports VoLTE

Difficult to be sure, but it looks to me that the FXS port is the only local telephony connection option and that can be used either with VoLTE, using the mobile operator’s PSTN connectivity, or using a SIP service, accessed over the air interface and then the internet. My impression is that it cannot connect to a local SIP implementation, and if one can it would be using it outside of its intended usage.

Details of how to set up SIP connectivity aren’t in the user guide, and have to be done through a remote web site, so it isn’t possible for me to work out if it would actually allow a SIP provider that is not over the air interface. However, it is fairly clear that SIP and VoLTE are mutually exclusive, so I’d say the only way of getting it to make VoLTE calls is with an analogue phone, and for SIP to SIP calls, you don’t need its telephony support.

OK so sounds like the modem needs replacing with something more suitable (some kind of gateway), or I need to use extra equipment to interface with it’s analog FXS port since I cannot directly connect to it without losing important features.

You may be able to use it, but only in data mode, with the telephony flowing from Asterisk to a third party SIP provider. What I don’t think you can easily do is use it with its mobile phone number.

Ah see now that is the crux of the matter. I would really like to use the sim card and phone number that belongs to it and not use any 3rd party services.

I’ve looked at possible alternatives and maybe something 2nd hand is the way to go. Something like a Cisco 819 4G LTE M2M Gateway perhaps.

Datasheet claims it supports SIP so it should be able to talk to Asterisk via ethernet and analog stuff should only need to be “local”.