How to forward voip traffic through a remote server?

Question

SIP

Hello! As you know, Beeline has its own sip telephony, which can only be used as a Beeline client (Internet or TV, it doesn’t matter).
In office #1 we have a beeline, in office #2 we have another (iota).
Actually I'm looking for a solution on how to use office 1 beeline telephony from office 2.
What is available:
In office No. 1, the Internet is set up with a static ip-address on the netgear jwnr2000 router, the router distributes the Internet to the server under win 2008 r2.
In office No. 2, the Internet is set up with a dynamic (temporarily, a static one will be received soon) ip-address on a mikrotik 951g-2hnd router. A panasonic kx-tgp500 sip-dect phone is connected to the router.
Voip traffic on Mikrotik is marked.
As I imagine: Beeline sip server settings (sip.beeline.ru and msk.sip.beeline.ru) are entered into Panasonic, during an outgoing call from office 2, Mikrotik catches access to these servers and conducts traffic through a remote server / office channel 1.
How can this problem be solved? Raising a vpn tunnel or what are the options?
I will be very grateful for methodical tips;)

Answer 1

If the beeline supports simultaneous registration of 2 devices, then this idea can take off.
The most beautiful solution without "perversion" is an intermediate SIP proxy server, any solution will do, even the simplest 3cx in demo mode with 2 outgoing lines :) or, of course, Asterisk ( freepbx ).
In exchange for a couple of days of hemorrhoids, we will get the ability to control communications, any number of devices, internal communications, transfers, call recordings, well, in general, all blackjack and mudmoiselles;)
Considering the presence of an external IP, the outcome can be used not just from office 2, but and from any smartphone with a soft dialer and normal internet :)

Answer 2

Install any router that can dd-wrt, and configure ddns on it and activate the milkfish sip router by reading the help on it.

Answer 3

1) to lift from vpn from the second office to the first.
2)
either prescribe the route "if we go to the beeline telephony server - we send it to the vpn channel", on the other side - just hit it.
or put asterisk in the first office and register by phone on it. The advantages of this decision (preference and mademoiselles) have already been discussed.
also, the two networks will merge into one.