Scenario: CME 7.0/ 4.3 - running IOS version 12.4(20)T using analog lines (VIC2-4FXO) on Cisco 2801 ISR
Issue: Calls to PSTN gets dropped after 10 sec; and you get No more ring descriptors available message from IOS.
Resolution: Found there is an issue with 12.4(20)T. There is a new release to fix bug, 12.4(20)T1