I have been a paying RBI customer for around 14 months I think and was part of the trial for around 9 months before that. I have had my 3G router changed, been pointed at different cell sites and had the yagi antenna checked.
It is not my IP address on the local side of the router that is changing. It is the IP address that the world sees for me.
The ip addresses listed below come from successive reload of http://www.whatismyip.com.
49.224.220.236 / 49.224.220.237 / 49.224.220.238