Open your PRI's DAHDI trunk configuration page and change the DAHDI identifier from gx to Gx (where x is your group number - so most likely from g0 to G0).
The identifier can be
gx - from the first channel up
Gx - from the last channel down
rx - round robin up
Rx - round robin down
Again, x is the group number - so replace it with the appropriate group - for example 0 -> g0, G0, r0, R0.