[tlug] VoIP help

[Kalin KOZHUHAROV <me.kalin@example.com>]

Hello,

While not strictly Linux, I am generally looking for linux-based
solution to an odd problem that had really drove me nuts for over a
week...

There is a device very popular in Germany (where I live now) called
FritzBox. Well, actually there are a plethora of somewhat similar
devices: https://avm.de/produkte/fritzbox/
The one I am dealing is [7390], for the record. It provides ISDN,
POTS, DECT, WiFi, NAS... (but has no beer tap).

The intention was to use it as a simple PBX with static SIP trunk,
provided by the ISP. So in ASCII, very simplified:

ISP --- router/fw --CAT6-- FritzBox :::DECT:::handset (x3)

[7390] https://avm.de/service/fritzbox/fritzbox-7390/uebersicht/

Problem is, that device does NOT support static SIP trunks (where the
provider does NOT require user/password or any authentication, they
accept all incoming calls from the static global IP address of the
FritzBox).
In other words, the FritzBox keeps sending REGISTER messages, which
the provider silently drops (per their documentation and actual pcap)
and refuses to dial out since it thinks no peer is available.
Well, at least it accepts incoming calls, so incoming calls are fine.

It took tens of e-mails, support tickets and few calls over more than
a week with provider and FritzBox manufacturer to finally understand
that such mode of operation is not supported "and thank you for the
feature request to be addressed in future firmware development" :-G

Since changing provider is kind of impossible, I am left with 3 options:
1. Find a device that will replace the FritzBox, say up to EUR300
2. Add some kind of gateway/proxy that will register the FritzBox and
transparently pass all calls to and from provider and can run on cheap
hardware
3. Add inline "packet injector" to return "200 OK" to any REGISTER messages

As you might have guessed, I am eyeballing number 3, hopefully using a
tiny OpenWRT-based box. Also considering IP spoofing / direct
injection (since provider silently drops the REGISTER). Any thoughts?

For 2, what kind of software do I need? I guess asterisk can be
configured in such a way, though I couldn't find similar deployment
setup.

For 1, I contacted TP-Link pre-sales last Friday since they have
similar devices, but have not heard back. DECT seems to be popular
only in Europe, so not many boxes provide it, and I have not found any
OSS for that (I am not talking about modifying DECT phones to
hard-wire a sound card).

Any comments and suggestions welcome!

(BTW, I'll be in Tokyo mid-December, someone may score a beer for a
good answer!)

Cheers,
Kalin.