Old iOS 10.3.3 iPad - FB messenger wont ring when screen locked

Hey all

Hoping to get some help here. I'm planning to give an old iPad to my grandma to be used for messenger video calls. When the ipad ipad running an old iOS 10.3.3 (the latest it can have) has screen locked, incoming messenger calls wont make a sound. However it rings when screen is unlocked.

Totally puzzled as I've gone through all the settings i can think of. It's not on silent, the ringer volume in settings is sufficient, notifications are allowed....

What am I missing?

thanks!