Hi Oleg,

Buy a domestic telephone exchange. A 1 to 2 line, one public line and two extensions connected to it.

You can ring eachother, ring out side and get ring selectively to both from outside.

Or, if want to do it yourself then focus on making such an exchange rather than asking otherwise.

Surf mitel.com for the necessary application notes and DTMF encoders and decoders. Use them all with the micro.

Be wise not to program the micro to generate DTMF, that you can do in 30 minutes and less than 2 $.

Have fun

Simon