"Can I increase the length of time the iPhone rings before diverting to voicemail?
Yes, just like any other handset you can change the length of time the iPhone rings before diverting to voicemail by entering **61*901*NN# SEND (NN is the number of seconds in increments of 5 and a maximum of 30 seconds)."
This code is incorrect. It should be "**61*901*11*NN# SEND"
I had a go at the code and also tried 25sec - it went to voice mail after 13 sec.
I think you mis-counted the "second".. I believe you've counted 1 ring for 1 second, but in fact, 1 ring = 2 seconds... So in your case, your call forwards to voicemail after 13 rings, just about 25 seconds, right?
iPhone 3G Black (OS 3.1) | HTC Magic (CM4.0.4) | BlackBerry Curve 8900 (Beta OS 184.108.40.206) | MacBook White (Windows XP Pro SP3 / OS X 10.6)