How to connect an analog communication line with a mobile one?

Question

Telephony

Good afternoon.
At the moment, the enterprise has one communication line, internal communication is organized through a mini-automatic telephone exchange.

The task is the following, when the client calls, the answering machine will start with a choice of whom to call, for example, press 1 got to the operator, press 2 got to the service center.
You also need to organize redirection to a mobile phone number (here I read about gsm gateways, though I don’t understand whether I’m looking in the right direction).
Line 2 must be mobile.
I don't understand how it works!
People, help with advice, I don’t understand at all where to look and what to do.

Answer 1

Asterisk?
Through FreePBX it is configured quickly and painlessly. The first point is solved through IVR and FollowMe, the second - by renting a mobile number from some VoIP operator, which the GSM gateway takes over.