Outbound calls

Hi, I have a working instulation on my raspberry pi with 4 google voice numbers setup. i am using a mitel 5215 ip phone, and it has 7 programable keyes, 2 are fixed as line keys the rest can be configured. how do i get line 1 to call out on the first GV trunk, and line 2 to call out on the second. is this a setting on the phone or on the PBX