Can't get old number out from a used phone

I've purchased a used tmobile g3. I'm running a custom rom / r rmx. I've tried to put my number in place of the previous owners' number. My number won't stick either in the phone or message apps.The phone was shipped with the stock rom and this is where I tried to change everything. Nothing I've done seems to work and I keep getting messages and calls from the owners number. How do I fix this??? :confused: