Change number of rings to answer on iRC3170 fax

Question

How do I change the number of rings to answer on the fax in my iRC3170?

I have been told I need to go into service mode to do this?

I want to change it to not answer until 5 rings of the incoming line. (I have a modem/winfax on a PC which answers after 2 rings to sort out the junk mail faxes)