Hi,
You could set a hotline from your branch to HQ using the command "connection plar" under your voice ports.
In this scenario you would pick the phone conected to one of the ports and that would ring a particular phone in the HQ.
eg :
voice port 1/0
connection plar 1000
dialpeer voice 1 voip
destination pattern 1...
session target ipv4 :
Configure the terminating gateway appropriately.This should help you.
Thanks
GS