I've got an 830 with 6 BRI-U cards. These feed data modems and telephone key systems. I've had a random problem for a while with it dropping calls unexpectedly. Now it randomly won't answer calls or allow outgoing calls. When calls come in, I watch the log and see that they come in, but immediately show dropped from the outside. The caller either hears nothing or get gets an intercept tone, then it hangs up. One of the modems says "answering incoming call" and freezes there until the ISDN line is pulled and put back in. I power cycled the 830 (it hadn't been power cycled in years) and the problem seems to persist. Also, then I power cycled it, I have been unable to telnet into it. I can only access it via the admin serial port. The problems happen on all lines across multiple cards. It looks to me like the main chassis and cpu have gone bad. I hate to buy a new one, the budget is tight. But, I am in a hurry.
For the telnet, first check and make sure the IP address is still set properly in the unit. This will be under system config, then Ethernet port. If the proper information is still in there then try to ping the Atlas and see if you get responses. When you say it randomly won't allow calls, is it allowing some calls and not others or are no calls going through now? Under System config go to event logging and set ISDN L2 Formatted to Info and try a call. Copy and paste the results into a file and attach it to your reply, please. I'd like to see if we see any errors that indicate a change on the BRI lines that could be causing problems.