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Also spend most time at home. The signal here drifts because of where the towers are. When the signal drifts the same switching phenomenon will occur. Watch your phone to see if your bars drop off and/or if what was 5g or 4g, has become 3g. After that I got nothing for a fix

Occasionally it happens because of reception. There are issues with some earlier Android devices like S4, 5, 7 that are this: the phone is supposed to switch from 4g to 3g, even down to 2g when signal for the faster data transfers is blocked or weak. Same issue for going from GSM back up to the higher data transfers. Android gets confused, stays on 4g or GSM and drops the incoming call into voicemail because it thinks the phone is out of range or off. Supposedly, if you frequent an area where LTE does not work well but GSM is fine, in Android settings under networks you can select either system to prevent the stumbled switching.