Organization of telephony in two offices?

Question

iPhone

There are two offices:
In office 1, a certain GSM gateway for 4 sim is installed
. In office 2, some surveys need to receive and make calls from the GSM gateway from the first office.
Also in office 2 IP phones and phones on iOS and Android will be used. I would also like to be able to forward some calls to Skype.
I know how to connect two offices, but I don’t know how to implement the work?
Help pliz

Answer 1

And what difficulties arise? I don't even know what confused you.
Office Router 1 take the "correct" one, OpenVPN quietly rises on OpenWRT, a route is prescribed that will wrap the entire traffic from GSM gateway to VPN. In the second office, you can symmetrically, or you can raise VPN only on a machine with PBX by forwarding ports to it.
It also depends on which gateway. On some, the same OpenVRT is spinning inside, so you can also lift the OpenVPN client right on the gateway. There are other options, but they are more hemorrhagic, because. NAT when setting up telephony is not the most pleasant.
Pay attention to the load - routers must have time to process traffic, otherwise it will not be comfortable.
Well, if you need to use the gateway in the first office too, put PBX in the first one, and in the second - as desired, or PBX (and connect them via IAX), or wrap IP terminals in the first

Answer 2

The GSM gateway is just a piece of iron between SIP and GSM, what is the IP PBX configured for in the first office? In fact, you need to connect the phones of the second office to this PBX, through which you decide for yourself (most likely VPN).

Answer 3

There is no ip PBX at all and it will be in the second office
How to connect a GSM gateway to ip PBX if they are geographically separated?