Muting / Trying To Facetime On Hands Free After A Screen Replacement?

Hi Everyone

Never had this one before. Replaced a LCD on a customers iPhone 6S yesterday and they sent me the following this morning. Would appreciate some help in diagnosing & resolving the issue.

"This morning I have made my first phone call since having the screen repaired and while on the call my phone (handsfree) it was muting/trying to connect to facetime and not allowing me to press any buttons to try and cancel. I noticed when I got off the phone call my phone was really hot and it had also drained a fair bit of battery on just a quick 10min phone call. "

All other functions/apps are working fine, it just seems to be an issue while making calls.

Anyone got any ideas? Front camera flex fault maybe?