How to make free telephony using a mobile phone?

Question

Telephony

Bottom line: I have SIM cards of three operators. Each calls within the network in the region are free.
I want such a mini-ATS (possibly with a gsm-gateway) to make free calls anywhere from my mobile phone from the office/apartment.
How to arrange it?
I understand what is needed:

1. PBX / PBX
   
2. GSM gateways for each SIM card
   
3. GSM transmitter for phone
   

The second question - let's say that N people will use such a system. How many transmitters will you need to have?
Thanks

Answer 1

1) The easiest thing is to buy a three-symbol card.
2) More difficult: Buy a computer + GSM-Gateway for each SIM card or immediately for 3 and raise Asterisk or 3CX and set up
routes try to run it.
The disadvantages of all this are that calls are not terminated on one number.
Pros: for this there is Asterisk where you can configure everything. The only question is time.

Answer 2

Asterisk + FriPBH + GOIP4 gateway, you can put a single-payer then the price tag will be about 20,000 rubles (gateway 13-14, single-board 4-5). Plug in 3 SIM cards (there are 4 slots), set up trunks, connect to an asterisk and specify outgoing routes using the mask 8 numbers of a megaphone ********** 8 numbers of a beeline ******, etc. Next, install the custom context module, assign a list of outgoing routes and the mask check order + permission to call on all trunks.

GSM transmitter for phone

what is this and why? Put some kind of zoyper + sim card with internet on your phone, forward the asterisk to the internet and receive incoming calls, call from sims that are in the gateway from anywhere in the world where you have internet. Believe me, this is the most reliable and cheapest option (if you want even cheaper, then this is an option with usb modems as a gateway, but they do not always behave stably).

Answer 3

I remember the Chinese did this in Moscow for "their own", so everything was confiscated from them - even in the news for 10 ... 15 minutes they told what and how.
Cellular operators are strict about this - the task of identifying the "smart" is their priority. Yes, and our laws in this regard can be "turned as you like."

Answer 4

1) There are devices called CallBack. Those. you call some number, and he himself resets and calls you back, after which you dial the desired number, but through the lines of this device.
2) You can also configure access through ATSku. You call her, add a pin code for authorization, then dial the desired number. Routing is configured on the ATS so that it directs the call through the desired SIM card. Work for 30 minutes, well, here the caller will have to pay for the connection to the PBX.
3) Everything is much simpler (on the part of the client) if this function is needed within a radius, for example, of an office. You can raise your microcell, connect mobile phones there, which will be considered as internal phones. But it is a little more complicated in terms of settings and finances.