Enable the use of Called ID and SIP extension to define
witch route the outbound call will make.
The proactical use of this is having a family group using
only one A@H, and each sub group (parents, sister,
etc) have their own pre-paid account with a provider, so
they controll and pay their own expenses.
The caller ID use of for the same reason.
If you have a single PSTN line coming into this A@H,
then you configure DAD's cell to go to A route -> X
provicer account. Sister's phone to go to B route -> Y
provider account, etc.