Voicemail: Can't Increase Number of Rings

I have tried many times to set my iPhone 5s so that the number of rings increases (from the current three) before voicemail is activated. I am using the correct code but always get the message 'Error performing request. Unknown Error. Setting Registration Failed. Voice Forwarding When Unanswered'. I have been to my carrier shop but no one there could fix the problem. All they did was reset the network settings which didn't work. I know that the code I am using is correct for my phone. I have used it several times with other phones. **61*901*11*30#

Does anyone have any ideas how I might solve this one without having to reset the phone back to factory settings?

Thanks

Graham