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Hi, I want to be able to deploy 2 PA1+'s and 2 VOIP phones for a simple PA use. I will set up a robust WiFi network between the devices.

The use case is for a funicular railway with a station at the bottom and top of the hill, each with a phone, and the railway carriages themselves to have a PA1+ each. The stations should be able to call each other and both should be able to contact the carriage PA1 - either single or on a group call. Operation should be simple using function buttons to initiate the calls.

 Is this possible and what are the pit falls?

Any advice appreciated.

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Simon Willcox

End user

Joined: 08.09.2025

4 Comments

  1. End user Jens Bürger

    You do mix up two things. Your thread title says "VoIP without Internet". Naturally, you don't need an Internet connection to do VoIP.

    To be more precise, your question maybe tackles whether a (public) SIP trunk is necessary to do some kind of internal calling. The answer also is: You don't need that either.

    But according to what I understand from your description, what you are trying to achieve is to do calling without some kind of PBX. In principle, that ist also possible, while not the typical way.

    What do you exactly mean by "group call"?

    Basically, VoIP phones (and the PA1 are internally VoIP phones also) use one or more SIP registrars they subscribe to. Routing of calls, i.e. "which phone is meant when someone dials extension 24" is typically handled by that registrar. If you register your phone at a public SIP (trunk) registrar, you typically have direct access to the public telephone network.

    For features like internal phone calls, like calling your local phones directly which do not need to or even must not be publicly callable, you need something with PBX functionalities.

    While you can use a "classic" PBX for that matter (given that it also can do VoIP), or many soho-routers come with a SIP registrar and some basic PBX functionality which should satisfy your needs. Since using it locally and without Internet access, a used / old device which does what you want it to should be enough. Since your post is in English, I assume that you are not located in Germany. In Germany, FritzBox is pretty popular regarding that matter and you could buy a used one which would satisfy your needs for maybe 20 €.

  2. End user Simon Willcox

    Hi Jens


    Thanks for taking the time to respond, appreciated & very helpful. What I want is to operate without using a SIP registrar or PBX. This because there will only be 4 connected devices. Since asking the question, I found the article below:

    How To Make Peer-to-Peer IP Calls without a Registrar - Snom Service Hub - Snom Confluence

    So, it seems that Peer-to Peer calls is really what I am after. That said, I suspect that calling on one phone to two PA1+'s at the same time would not be possible with Peer to Peer (this is what I meant by group calling).


    Thanks again for the pointers.

  3. Snom Jan Boguslawski

    Hi Simon, you are right, the Peer-to-Peer article is what you are looking for, plus I do see a potential "workaround" regarding your group call need:

    Never tried it for Peer-to-Peer calls, but maybe you could leverage the Ad-Hoc Conference feature:

    Ad-Hoc Conference - V10

    in case the IP phone supports this also for IP to IP calls (no registrar / PBX) - the phone will host the Conference (mini-MCU) - allowing you to talk to both PA1+'s same time.

    As both PA1+'s will (default) auto-accept any incoming call - the incoming Ad-Hoc Conference invites (phone will send both SIP Invites in parallel) will be instantly connected to the mini-MCU.

    I don't know atm. exactly how, - but I expect / guess that the workflow of the Ad-Hoc Conference - V10 could be re-created in form of a hardcoded Ad-Hoc-Conf-XML file (hardcoded in terms of participant 1 and participant 2 contact / IP address) and finally programmed as a XML defined key to one of the many phone buttons...

    In case, my assumption is possible - you would have just one button to press as a user to call both PA1+'s in parallel - your "group call" - even without a PBX / registrar...


    At totally different approach, but maybe even easier could be the use of Multicast Paging: How To Configure Multicast Paging

    In this scenario, the PA1+'s would be configured to listeners of the Snom phone's multicast. On the phone you would just programm a multicast key and when pressed you instantly talk to the PA1+'s.

    Only limitation to keep in mind: The Multicast will not propagate across Subnets and VLANs.

    btw: combining M-cast with Peer-to-Peer / IP 2 IP calls would also work. So in case you want to call just one of the two - you could programm 1 or 2 buttons with the contact (IP) of the corresponding PA1+

    Hope you will find this info supportive.

    Thanks, and greetings from Berlin,
    Jan

  4. End user Simon Willcox

    Hi Jan


    I find that extremely supportive! Thanks so much for your help, very useful pointers.


    Kind Regards


    Simon