o2  ... how do I change the length of time the phone rings before ..

.. the woman comes on saying whatever it is she says about no answer.

Here's the situation:
My friend and I and her friend and her mother are all on o2  (4 people ;-) 4
o2 phones)
my friend rings me, her mother or her other friend and her phone makes only
six rings her end before the o2 voice stops the call.

I phone my friend or her friend phones her and our phones allow at least 14
rings before giving up.
My friend and I don't have voicemail activated, but her other friend does.

When my friend phones a land line it will ring till the end of time ... (I
did not test this till the end of time but it rings a long time.

We want to increase the time her phone takes to give up ringing out to other
o2 numbers

any ideas?
code_wrong