There is an open defect CSCse04439, however I think this would impact all users that try to change the phone number. Here is the information on that defect, if you don't see the DCOM errors in the system log, it's likely a different issue:
Symptom:
When using the Unity Mobile Message Access, you enter your pin and the number to call then you receive a message stating:
Unable to change#
Unknown error 80070005
You will also see DCOM errors in the System Log which have a description similar to:
machine-default Local Activation {60EB7702-6A7A-11D1-8029-00C04FAD610B} IUSR_
Conditions:
Unity 4.1(1) or later using Mobile Message Access
Workaround:
Go into Component Services and give IUSR_ access to AvCsGateway.
As for the issue where the one user's phone never rings...there have been issues reported with some (older?) Blackberries where the device does not allow simultateous data out communication and phone call in. So the call goes straight to voicemail and never rings the phone. You can use Port Status Monitor tool to see if an outdial is generated, and if so whether it is sent to voicemail.
