Try this, to change the number of seconds before it goes to voice mail, you'll need to dial: **61*+44yourvoicemailnumber*10*amount of seconds#
Where 'yourvoicemailnumber' is your phone service providers voice mail number. To find the voice mail dial number Touch the 'Dialpad', then Enter *#67#, followed by touch 'Call'
The amount of seconds are in increments of 5, with 30 being the max. So if you wanted say 15 secs, then you would dial:
**61*+440717941123456*10*15# (replacing 0717941123456 with the voice mail number you obtained previously)
The amount of seconds are in increments of 5, with 30 being the max.
Hope that works for you.