First, each seat that wants to be able to receive calls has to enable their in-seat phone. This is done by turning on the phone, and then pressing [FUNC] + 2. The phone should then display "CALL RECV ENABLED". The phone can then be turned off and stowed as normal.

Second, to place a call to someone who has enabled their phone, turn on your phone and press [FUNC] + 1. Then, you can enter the destination seat as a two-digit seat row + seat letter.

For example, if you want to call 27C, you enter 2 + 7 for the row, and then hit 2 three times to get 'C'. The recipient's video screen will then pop up a message about the incoming call, and they can answer the phone.

For more information, please see this tread:
:-: Hacking the 767/777: Making Seat to Seat Calls :-: