If you want to switch of your voice mail dial 1760. Switching vouice mail back on you need to dial 1750.
If you want a miss call alert (So you receive a text to let you know that some one has tried to dial you and you didnt answer, you wernt in range, phone switched of or you was on another call then dial 1710 (Remember changing to miss call alert will automaticaly switch voice mail of.
If you want to extend your ring tone then dial the followingf *61*901**30# The above code will extend you ring tone to aprox 30 seconds which is the max
Hope the above helps
Iphone 5 * SE T68i * On and On * 2GB Data * O2 Wallet * O2 Legacy Broadband * TU GO