Ok sounds pretty bad.
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Do you have an SD card and is it formatted as internal, if so that could be an issue?
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You may require new SIM cards. SIM card can get tarnished and make poor contacts. and/or if they don’t fit well i.e. if they bend a bit in the sloe you’ll have poor connections.
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You may have poor aerial connects which will require a dismantle and clean to check
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Do you still have any of the 2 year warranty left?
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You can try running in safe mode that disables custom apps and may help narrow down a problem
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Finally you can do a factory reset but you had better save all your personal data , numbers and image etc. as if will wipe the phone, which is the first thing support will do if you send it to them.
You haven’t said where you are of what network you use so that someone can compare
You haven’t said what OS and update you are using.