"How do I change my voicemail settings on an iPhone" is a question we often get asked on Macworld. Especially how do I stop my iPhone from going to voicemail after just three rings.
There seems to be no settings for voicemail on the iPhone itself, and this is because your voicemail isn't controlled by the iPhone itself, but by your network provider. You have to adjust these settings by dialling your network and using a phone option.
First of all there are two things you can check if you're having problems receiving calls.
Ensure that Settings > Do Not Disturb is currently set to "Off." If it set to "On" then people have to call you twice within three minutes for the call to come through.Go to Settings > General > Reset and ? Reset Network Settings. This will ensure that you have the right settings for your network.
Whether you can change the length of time that the phone rings before going to Voicemail depends on the network provider. Some, like Vodafone, enable you to change this using their automated system. Others require you to call up, or the amount is fixed.
See: Hands on with iOS 6: Settings Change the voicemail ring time on Vodafone
Vodafone, for example enables you to length of time a phone rings before going to voicemail. According to the Vodafone instructions:
Enter **61*121*11*[Number of seconds]#
Then enter the number of seconds you'd like your phone to ring for (it has to be between 5 and 30 in a multiple of 5), followed by #.
For example, if you want to set if for 25 seconds, you'd enter **61*121*11*25#, then press call.
Vodafone will send you a text letting you know if this has worked.
Turning voicemail on/off with Three
To adjust voicemail settings (including turning voicemail on or off) you need to call three on 333.
To turn off VoiceMail on the Three network dial "333" and tap the "Keypad" in the Phone app. Press the the following buttons
You should get a text from Three stating that you have succesfully turned off voicemail. To turn it on again go through the steps but change the last entry to "2".
Turn Voicemail on/off with O2
O2 has a direct line to turning voicemail on and off. Just dial the following numbers direct from your phone.,
Turn voicemail off: 1210 Turn voicemail on: 1750