Why do you think so?
Why does DebFrmHell think the calls on 3/2/12 were made by law enforcement? Well, they had the phone and the calls were made just after a 911 call from Martin's phone. Singleton's certainly a possibility for the 911 caller given the statement you just quoted:
On March 2, 2012 Inv. Singleton contacted It representative from T -Mobile who told her if the Sanford Police Deportment obtained the cell phone number and the pin number to the account they would be able to bybass the swipe code on the cell phone.
On the very day Singleton called T-Mobile, someone called 911 from the phone, which is clearly an attempt to obtain the phone's number. Then a few moments later, someone calls the phone, which is clearly an attempt to confirm the just-obtained number.