My son's cell phone consistently gets into a mode where it cannot receive calls for a period of time (straight to voicemail). It basically always happens within an hour or two of first noticing that everything is working fine. During this "dead" time, it can send and receive texts and use data. If I make an outgoing call on his phone, it will always work and then suddenly he can receive calls again (for the next hour or two). Also, if you go to airplane mode and back, or switch off/on wifi, something magically flips inside and it can receive calls again. If you just wait it out, it eventually (don't know how long exactly) will cycle back and work again for a couple hours.

He is using my wife's old Samsung Galaxy S7 Edge that was factory reset back in November and has been used as a wifi-only device up until Monday. We signed him up for Mint Mobile and received a sim card from them. We unlocked the phone from the AT&T network and successfully connected to the Mint service. Everything else works fine except for this regular glitch regarding receiving calls.

I've tried different APNs, turning on/off advanced LTE, clearing data and cache in phone apps, messing with battery optimization settings, restarting a million times, explicitly disabling any call forwarding, testing in various locations, etc, with no positive or negative effect on this problem. Some comments online talk about how Mint calls on T-Mobile network can be dropped due to congested cell towers, but this happens any time of day, and consistently within a certain amount of time of the phone being "up". To me, it feels like this is probably either a faulty sim card or there is something with the phone's battery optimization that is impacting the ability to receive calls until it gets a trigger to restart - but I really don't know.

Any ideas? Have you seen or heard of this before? I've got a replacement sim card ordered, and the next thing I would try is a factory reset.