Adding to what Westi said. When your calls drop...do your displays on your 18ds or 34ds go blank, then your get a whole bunch of flashing red lights throughout the phone. I have seen this problem way to many times to count. Each time the problem has varied from a defective processor, a defective 308 card, a defective UPS, a defective carrier once, and even a bad channel bank coming in from a Clecs T1. The best way to start is to notice what is happening on the phone when the drop happens. The better thing to do would be to dial into your system and look at the maintenance log. You would be looking for a high amount of either cold starts or warm starts. Or you could place a camera on your ACS and see if any of the green lights go out. I hope this helps.
A.J. Wiesner
AT&D Communications