How to organize telephony between branches?

Question

Asterisk

Hello.
There are 7 branch offices (average 10-15 people per branch). Between branches there is a VPN
To reduce the cost of telephony, I decided to make an IP ATC based on Asterisk.
The idea is to transfer calls between branches via VOIP, as well as "attach" corporate calls to internal numbers. Since we mainly have analog phones, VOIP gateways will be purchased.
At the moment, telephone services are provided by providers in their region.
The question is:
1) Is one Asterisk server in the main branch enough, or should I install a server in each branch and make an IAX trunk?
2) If there is one server, how to steer trunks in branches? Specify them on each gateway or on the Asterisk server itself?
3) In the event of a network failure in the main office (power outage or problems with the Internet provider), then other branches remain without telephony (both internal and external). How to make a backup channel so that there is internal and external communication?

Answer 1

Ideally - the most unified solution. Each branch has its own Asterisk, VoIP gateway to the operator, SIP phones to workplaces, IAX trunks with other branches. In this case, disabling channels in one office simply switches it to offline mode, the ability to call to the city and receive calls from the city is preserved.

Answer 2

as well as "attach" corporate numbers to internal numbers

It's not entirely clear what this means. Corporate mobile?
1. One server is probably not enough. Point of failure.
2. Yes, trunks are configured on the server, and gateways connect to it.
3. The question goes beyond telephony, you can probably buy 3G routers.
In general ... now Asterisk can be deployed even on Mikrotik, it is inexpensive and quite suitable for small offices.

Answer 3

I think one Asterisk is enough i.e. it won't be a bottleneck. Quite successfully it turns out for them to steer trunks (verified by experience).
For dynamic call routing between Asterisk hosts, configure DUNDi voxlink.ru/kb/asterisk-configuration/Conf_DUNDi_As...