Have iPhone 5 currently on ST. Persistent problem with dropped calls, sometimes several on the same call to a landline. Searched this forum and others, applied suggested remedies to no avail. Replaced SIM, reset network settings, hard started phone, consulted with Apple etc. One solution is to return to ATT although ST appears to be on ATT network. Phone is in major metro area with no problems in this regard. Any assistance or guidance much appreciated.