Can't call certain phone numbers??

[Guest_imported]

Currently we have a Mitel switch with a PRI connected to a Cisco 3600, which in turn has a PRI connected to the PSTN (Sprint) for our phone service in our office. We are using Mitel IP phones in the office. We also have customers that make calls over our Satellite network, which connects to the Cisco 3600 (not the Mitel) and out the same PRI to the PSTN as our office. Our customers are having no troubles at all (thank god!!!) but our office is having some issues. We are unable to make certain calls out, including local and long distance. We cannot call any 18XX numbers, and various local numbers as well. There is no pattern to the numbers we cannot call, ie. area code or local codes. Keep in mind that we can make some calls no problem, and there is no pattern to the ones we can make. Does anyone have any suggestions?

Dave