If you've unlocked your iPhone via the AnySIM application or another method and have lost your ability to access visual voicemail, there's an easy fix -- as reported by iPhone Atlas reader "Gumps," and originally posted on on the whirlpool.net.au user forums by Simon K.
"Many people including me had an unusable voicemail button on the iphone after unlock. I have found a simple solution thanks to whirlpool.net.au forum's user Simon K."
Open the Phone application, then tap the keypad function. Enter the following:
*5005*86*phonenumber# (where "phonenumber" is the number you dial to get to your voicemail) then press the Call button.