Really old stuff, but just for fun.
I remember this very clearly. Several engineers used several weeks to trouble shoot why ISDN modem wasn't working.
They did reset the modem, and try Internet connection, working.
They did reset the modem, and try credit card authorization connection, working.
But when you did those in serial a modem stopped to respond.
What they didn't get, was that the modem mode needed to be correctly configure simply using one hayes command to change the mode. Instead of that they tried everything, installing multiple modem drivers in parallel, configuring those using the modem drivers user interface. As well as in some cases installing two parallel ISDN modems to computer, because 'one didn't do it and didn't work'.
After changing that just one parameter in configuration file, everything worked just fine. You could use it for Internet connections, banking traffic and for other uses too like credit card authentication calls. No need to install several parallel modems, one for each use.
What did they miss?
One command changed the modem response mode (V0 vs V1). Which changes what kind of return strings (number codes or english messages texts) the modem returns after each command. If the software wasn't able to handle both or mode was incorrect, of course that modem appeared to be dead to other apps trying to use it.
Sometimes I'm really baffled how incapable people are solving problems, because they don't want to dig into the problem. Instead they try over and over other simple silly workarounds which won't work. Same applies to restarting programs when those fail. No, restarting won't ever fix a real problem. Instead you should look for the real reason why program isn't working as expected.
Another fun BBS related story
I changed my modems escape command from +++ to ''' after that I was able to issue +++ command, which of course was echoed by the other end to their modem. Using this echo trick allowed me to issue configuration commands for their modem. With some BBS systems it was possible to logoff and logon again, without acutally disconnecting and now gaining new full session with fresh connection time limit. Using this trick you could also potentially create a Denial of Service by reconfiguring modem so that it doesn't answer calls or give indication of incoming all so that the system would detect it and answer to call. - Those were good times.
Wikipedia: Hayes command set
Of course everyone should have tried every command as well as vendor specific commands and experiment how those affect things.