Same problem here,
I called T-Mobile (150) and they confirmed number of rings cannot be amended from the phone, it is a network setting, she gave me a long code (about 16 chars I think) that I had to enter into the dialler and call. Phone now rings 5-6 times before being forwarded, much less frustrating. She did mention that each code is unique to each phone, so no point me reciting it here. and she confirmed any future network updates may revert me to three rings again by default so I would have to re-enter the code, therefore I have added it as one of my contacts.
Hope this helps
Dave