Part of the problem is defining your threat model. For example, if you're trying to avoid surveillance dragnets like geofence warrants, that can reasonably done

Part of the problem is defining your threat model. For example, if you're trying to avoid surveillance dragnets like geofence warrants, that can reasonably done by GrapheneOS+prepaid service+ cash-bought phone, combined with keeping your phone turned off near home (separate device for home use). It's also best if your mobile phone number is never used or known by anyone because you use a VOIP number (further decouples your mobile tracking identifiers from your public persona.) But yeah, at some point you have to accept that you can't be zero-trust with your mobile provider. Edited to add VOIP recommendation.