Shortest way GSM->Zoiper?

Question

SIP

Good afternoon!
I got a problem: to organize ip telephony for 1-4 employees with a minimum of rules - simple switching, preferably with SMS.
I immediately offered one of the cloud services, already registered the numbers, but an addition to the TOR came - I need control over the equipment.
How I see it:
GoIP-4 gateway ----- what is it? -------------------Client
<---???--->{Internet}<---> [iPhone(Zoiper)]
Further my knowledge is not enough, I ask for advice.

Answer 1

GoIP-4 gateway ----- what's here? -------------------Client
here PBX, for example Asterisk