This is my understanding of how it works.
Scenario 1:
IP phones (A and B) -- CCM -- Gateway -- PSTN -- Mobile phone
If phone A calls phone B and phone B forwards call to mobile phone, the RTP stream is between phone A and Gateway.
Scenario 2:
IP phone (B) -- CCM -- Gateway -- PSTN -- phone A and mobile phone
Assuming phone A is an analog phone or another mobile phone. If phone A calls phone B and phone B forwards call to mobile phone, there is no RTP stream once the call is forwarded.