Why is phone trying to make international calls?

[AC Question]

I am a cell phone dealer and am having issues as of recent when I sell a unlocked phone to a customer. Usually is a verizon cell phone and they have a different carrier and adjust their apn settings correctly. For some reason calls will come in but with a "+" in front of the number and say they miss the call and try to click the call log to call back it tries to dial internationally and won't work. I've tried everything I've known but can't get this issue fixed. Also went into call settings and into the addition e dialing and such and tried to change dialing numbers from 11 to 10 and also removed the +1 from country code. I don't know what else to do