Voice Mail Box not setting up

[VOL559]

Trying to set up my wifes voice mailbox and after we choose default greeting we get this box that reads
"Voicemail Unavailable
Unable to connect
Try again later"
Any suggestions ?

 

[crowd pleaser]

Call your air time vendor, usually 150 from your affected phone. Likely you have the wrong number set up.

Also some versions of iphone software update meant you could not set call forwarding up yourself.