I had this problem with my iphone. Always a mad scramble trying to answer phone within 16 seconds before it went to voicemail. I rang Sky and they instructed me to tap a code into the phone's keypad and how many seconds I wanted it to ring for. Then you tap the green phone icon. As simple as that, they said, but it made no difference. I spent hours and hours trying to solve this problem. Eventually found the answer -
On a recent iphone update they introduced a new feature called Live Voicemail, which means you can listen to the message while it's being recorded and answer the call if you choose, instead of the old style voicemail where you can only hear the message after it's been recorded. The Sky code to extend the ring time will only work if you turn off this Live Voicemail feature. Go to Settings and scroll down to the bottom and tap Apps, then Phone, then Live Voicemail, turn that off. Now, your phone will ring to your chosen seconds and record the old style voicemail message, where you 'dial' 751 or 759, or whatever it is, can't remember, and the BT-style-robot-voice will lead you into the message left for you.
It seems to be a problem that has stumped a lot of people, so I hope that helps..
