PINs are not accepted from particul area

When my calling cards customers of some particular area enter PINs from their telephone, are not accepted by the originating GW AXT800, and hear prompt to try again and again. But at the same time customers of all other areas are not having such problem. I tried to solve this problem by changing parameters of Impedence (0-15), but restult always same.
I appreciate suggession to solve this problem from experts. Thanks.