What is your PSTN connection? SIP, PRI, or analog?
If no changes have been made to cause this change in call handling, one physical defect that might possible create these symptoms is the loss of the DSP's. To check for this, telnet or ssh to your 7100 and "show version" -- Right after the system up-time, you should see something like this:
slot 0, DSP 1
DSP software version: G2.R10.8.M.4b
DSP hardware version: Freescale MSC7119
Total channels: 30
If you don't see this, then that indicates that the DSP's are not detected, and the unit will most likely need to be replaced to correct this issue, since the DSP's aren't field-replaceable. If you DO see the DSP's [and no config changes were made to alter call-handling], then you should start off with some debugs such as: debug voice summary and debug voice detail.