Weird fault on t610

Hi,
Hope somebody can help me.
My T610 works fine most of the time however problems start if I ring
somebody and their phone is engaged.
After hanging up after an engaged call, every subsequent call returns
'Number unobtainable' tone (vodaphone network UK). The only way to clear
this is to power the phone down and back up again.

I know that the 'NU' tone comes from the telephone exchange and not the
phone (trust me I'm a telephone exchange engineer), therefore I assume that
the phone is actually dialling out rubbish - despite showing a legitimate
number in the window. Is the phone on it's way out or would a reload of
software help and if so, how do you go about it?

TIA

I, MM