How to forward VOIP handsets from another office?

Question

Telephony

There is an office with VOIP telephony. Recently, a branch was opened for 2 employees in another part of the city. They moved, connected the Internet.
Now the question arose of how to bind them to the main office (only applies to phones). The server is made on Elastix. I understand that it is necessary to forward the tubes, but I don’t understand what goes where.
Here is the diagram:

Answer 1

@lamonteen
vpn to raise the channel between offices is the most correct solution.

Answer 2

I did this: on the router at the head office (with a white external ip 1.2.3.4) I opened ports (5090) and set up forwarding to the elastica ip (let's say it has ip 192.168.1.2:5060). in VOIP handsets in offices, specify 1.2.3.4:5090 as the server for connection. of course, you need to take care of security (white lists of incoming on the router, at the head office)
UPD: and don't forget to forward RTP ports too