Treo 650 on Cingular accessing a Cisco voicemail system.

Has anyone had any sort of intermittent trouble accessing a corporate voicemail system with the 650? When I enter my mailbox number I frequently get the message, "You currently are calling from a phone that is not configured for voicemail on this sytem"
After checking with my IT group they said the voicemail is fine but something is happening either on the phone or on Cingular's network.