Issue with incoming calls

For the last several weeks when my friend tries to call me, the calls will not go through. She says a recording comes on saying this number isn't accepting calls at this time or the call can not go through at this time. She is able to text me and I can call and text her. Her number is not blocked and do not disturb is not turned on. They are both iPhones. My phone has not been off or out of service and it only has been an issue with calls from her and no one else. What could cause this? And how can it be fixed?