How to implement SIP/Voip between two offices?

Question

Yota

Good afternoon!
I will make a reservation - in this question I am a complete zero, but still ahead :)
There is such a task - to implement the interaction of two offices by means of a telephone line.
There is office B, it has Yota + Zyxel Keenetic 4g II + D-Link DVG-5402SP + two phones.
There is office A, it has only 4 mb / s Internet.
UPD: There is a VPN between offices.
Four people sit in office A (each should have their own phone), they need to periodically contact office B, and office B with office A. They interact only within two offices.
The question is, what is the easiest way to implement this? What do you need to buy in office A? To what side to look?

Answer 1

You can start by choosing devices - these can be IP phones, PCs with headsets, smartphones with the Voip application, the voip application on employees' personal smartphones.
Further, it is highly desirable to have a white static ip
Between offices, you need to organize a VPN
. In one of the offices, it is desirable to organize a voip server, for example, asterisk.