You cannot place a test call over a PRI trunk. You MUST make sure you're interface ID's match the service provider. They should be able to tell you which circuit (by circuit ID or DHEC etc) is which interface 0, , 2, 3 etc. Some vendors skip interface 1 if there is only 1 d channel.
Another thing I have seen cause this problem is a bad medpro board. Verify you're medpro boards are in the correct network region.
You'll most likely have to setup a time to troubleshoot ... tell all of the agents to raise their hand when they get a bad call or one way audio. Tell them to keep the call active as long as possible and make sure to be ready and have someone relay their station id so you can get a status of the station.
Each time you get a bad call stat the station and record the CLAN, Medpro, and trunk resources the phone is using. Start building a table of bad calls. If you want you can also do this for good calls to help balance the info. If its a medpro you'll start to notice that all the bad calls seem to be occuring on a specific medpro board.
If you find that to be tru, campon-busy0out that board then see if the problems go away. If they do you can try reseating and or reseating that medpro but I have found that doesnt usually work. I support 11 S8700 and hundreds of Medpro boards and I see this problem about once a month.