I'm looking to use a dialup connection as a backup and had some problems with 5.203 so backed up my configuration and gave 5.900 a shot (also tried 5.841 so it's not specific to 5.900).
However, the modem never gets activated and dialing never occurs. The serial port is detected normally, but nothing happens when activating the interface.
Let me know if there's any more information I can provide.