your "dial" command can specify the "Group" to dial and the pri channels can be organized into groups.
the dial command should not do this directly, of course, but using a "global variable" so that this can be used in more than one carrier.
while the "carrier" settings are not generally used for Sangoma/Digium cards, that does not mean you CAN'T use the carrier settings under admin for this purpose.
so you can create groups withing your pri setup following standard asterisk setup (directly in the .conf files), then you will USE these groups in global variables defined in ANY carrier in the "Globals String" (or directly in the extensions.conf file). Remember that once defined, they can actually be used in any carrier (not limited to a specific carrier, even thought it would appear to be arranged that way).
then you use those global variables in the dialplan entry on the "dial" line to specify which group will be used for this dialplan (and then you end up with a different "group" for dial 9 vs dial 8 vs dial 7).
Very similar to how the rest of us would use the admin->carrier settings to choose between different sip carriers.