DMS,
Are you telling us that you work in a central office on a DMS(100-500) I would imagine and you have a comdial system(DSU) that has trunk lines that come from the DMS(or what ever CO switch you have)...deep breath....and you are trying to dial into the voice mail that the CO switch is connected to.
Can you dial a 7 from any phone. Have you tried it at another station? Did you try replacing the set at the troubled location