Im on ATT but my phone keeps locking onto a petrocom tower even when a ATT tower is available. I have found that the petrocom network is on the preferred networks list. ATT charges roaming for this so i find it odd that it is on the preferred network list. I want to put it on the forbidden network list. is there a way to edit this (its on the sim card) from my phone?? its costing me an extra 100$ a month.