Iphone Tmobile Plan

I just unlocked my Iphone and have Tmobile with a blackberry plan set up. I was told to switch from the blackberry plan to the tzones, now called web2go, plan so that I could save some money. When I called they wanted the IMEI number so I told them I would call them back. I can't change the service online and worried that giving them the IMEI they will know that I have an Iphone. Should I be concerned about giving them the IMEI and getting in trouble for having the iphone? Thanks.