I don't understand what you mean by when you dial 172 it changes to 171. Please explain exactly what you're doing and what happens. If you mean that when you dial 172 it actually dials 171 then that's something weird happening on your phone and nothing to do with the network.
Did you follow the instructions for setting up voicemail?
Strange. I dialed 172 last night and it also terminated more or less immediately but this morning it gives me the menu options for recording a greeting etc. Have you tried again this morning in case it was a transient problem.