Здравствуйте! Попробуйте так:
Настройки freepbx
Trunk Name: mypbx
PEER Details:
username=freepbx
type=friend
secret=mysecret
qualify=yes
port=4569
host=dynamic
context=from-trunk
trunk=yes

Настройки mypbx
Provider Name : freepbx
Hostname/IP :[ip адрес freepbx]
Username :mypbx
Fromuser :mypbx ...