Here\'s my take...
I assume you are pressing the numbers correctly.
The one possibility I can think of is, that there is something wrong with the dialpad on the phone. Try dialing from a different phone.
One case... My friend was using an imported(he bought from Japan) cordless phone. That phone did not transmit \'#\' or \'*\' characters properly to the base, and he could never use it for certain things. He could use it only to make plain voice calls.
Just a thought. Let us know how you make out. Good luck.
sammy