How to set your voicemail button to another provider?
The iPhone’s voicemail button is considered pretty much useless if the phone’s not activated via AT&T. Afraid not, you are able to assign it to any provider’s voicemail service by following the steps below:
1) Obtain your provider’s voicemail number
2) Click the phone button on your iPhone and access the dialpad.
3) Type the following code onto the dialpad *5005*86*xxxx# (where xxxx = voicemail number, so for an example, Singtel’s voicemail to be set on the iPhone will be *5005*86*1389#).
4) Depress the call button.
The voicemail button has now been asigned to the respective number of your service provider. Tap onto the voicemail button on your phone menu to try it out.