Problem
My Telus iPhone 4 didn't have the Voicemail button programmed. It would just spit up an error saying
Cannot Connect to Voicemail
Call your service provider for assistance
Solution
From the keypad, type
where the dashes represent your 11-digit voice mail number (typically starting with 1). Then tap call. The screen will flash, ever so briefly, and after that it looks like nothing happened. (It doesn't actually make an outbound call.)
Now the voicemail button works.
(Source:
planken.org)