Unfortunately cellular coverage is not a absolute science. Many factors can cause poor signal, scratchy calls or slow data speeds. A simple electronic signal from radio and TV towers, building materials or terrain etc. The lower frequencies are what get through to buildings not the higher bands. It is quite possibly that Sprint uses a different signal I know they use a different technology even though the like Verizon use CDMA it is not comparable.

If service worked prior to you joining verizon wireless, why did you switch? Did you even check out friends or coworkers in your area if they had stable service.

In your case the not verifying coverage and strength of signal has just wasted your money. It is a shame that you now must return to Sprint and lose those Early Termination Fees.

Good Luck