iphone custom voicemail greeting won't save

when i try and set up a custom voicemail greeting, i hit save and after about 30 seconds it times out and says "error: voicemail greeting cannot be saved."

it's not a mic issue because i've pressed "play" each time to listen to the greeting first.

i've rebooted the phone and reset my network settings, but neither has helped. i'm on rogers in canada if that possibly makes a difference