Have an unlocked Verison Samsung S4 Model SCH-1545 on AT&T that would not receive calls. Could make calls, send & receive text messages, just could not receive calls; wifi works fine. However, finally found a solution.
In order to receive a call, the party calling must be in the yellow "Contacts" app AND the telephone number must be specific whether it is a cell phone or home phone number. If the type of number is not specific, the call will go to voice-mail.
When a call is received and the caller is displayed on the screen, slide the caller to the green phone icon near the lower right hand corner of the screen to accept the call.
Pressing the "Home" button will not answer the call.
In order to disconnect the call, simply press the power button on the right side, OR press the "Home" button (if the screen is blank) and then press the red phone icon near the lower left hand corner of the screen.
Have no idea why the above worked and makes no difference whether the phone is rooted.
One last note. Depending upon the telephone service provider of the caller, a "1" may be be needed before the area code. Example: Consolidated home service which is a VOIP, requires a "1". However, T-Mobile does not.