Calls and texts work intermittently

Refurbished iphone 6. Calls and texts work normally less than half of the time. Even with full reception it says “call failed”. Texts seem as though they are going through but the blue line just sits halfway across the screen. They don’t even send when choosing the “send as text message” option. The SIM card has been replaced. I’m wondering what trouble shooting I can do? Thanks in advance :)