iPhone 7 issues

My iPhone 7 which is around 15 months old has started to have issues and is essentially unusable.

When an incoming phone call is picked up via swipe the phone keeps ringing / vibrating, outgoing calls do not end up being up properly shown on the screen or heard (the person on the other end will get the call), the device takes anywhere from 30 minutes to 4 hours to boot up.

I've tried reset settings, restore phone, hard reset and have taken in to the genius bar (and did a reset there) to no avail.

I did not drop the phone or in any other way damage it nor did i download/update anything before this started to occur including iOS updates.

Please help