phone locking every time we have to pressreturn key and volume up

In my experience can be 3 differents things:

1. The volumen flex is bad

2. The power button flex is bad

3. Water on the board

The best way to test this is opening the phone and disconnecting the volumen flex from the board and check if still is not working properly.

Do the same process with the second flex, if still the same problem probably water damage but almost always is one of this two flex.

In all the phones or tables is the same process, the difference on same models is that sometimes the flex are solder on the board or is a contact flex but the idea is always the same.