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I don't have a flip phone but I have experienced callers going straight to voicemail. For me, it was the scam blocker flagging people incorrectly causing the calls to go straight to voice mail. However, that most likely would not apply to you on a flip phone. If I was you I would go through the process of eliminations. First, try to identify when this is happening. I.e is service strong when it occurs or weak? Is this happening in a set area(indoors at home or anywhere), and so on. You can also call into T-mobile and speak with the tech team to see if there are any reported issues for your area.
I'm confident the network isn't messing with your phone, and I'm sorry you're having these issues. Calls going straight to voicemail is normally a signal issue either in a particular area or something going on with the device. With that being said, the best place to start using our Signal issues / no service troubleshooting to see if that helps with signal and improves incoming calls.