You would have to have someone who can duplicate the issue open a ticket with their local ISDN provider to trap their call and see how it is coming, a SW64/56 data call or voice. If it is coming from the originator as a data call to the local ISDN provider then they would have to trace how they are routing it to the destination, whether they are sending it over data trunks or voice trunks. It could be the call is traversing voice trunks somewhere along the path either from the local ISDN provider to the long distance provider, across the long distance providers network of switches or the handoff from the long distance provider to the terminating local ISDN provider (yours). Are all the failing calls from the same long distance provider? Support for SW64/56K ISDN calls has been reduced/phasing out by long distance providers so that may be affecting certain originating callers routing of long distance ISDN data calls. They may be able to receive them (from you over your long distance carrier network) but cannot originate them.
Last edited by Joe Vallender; 04-16-2016 at 08:39 AM.
Reason: additional info