Recently upgraded to handheld software is v.18.104.22.1689 (latest available from T-Mobile). Uneventful upgrade. Everything appeared to work.
Seems that ability to make conference calls was lost immediately after that, although could be coincidental. Error message after pressing 'Join' (as in join two calls) is 'Operation Failed'. Conference calling used to work like a charm, and was a frequently used feature.
Any suggestions for possible remedies? T-Mobile support folks don't have any ideas.