Pick up phone to make a call and there's already someone on the line

Unsure where to even being hunting down this one.

Some staff have experienced an issue where they pick up their handset to make a call and immediately have an incoming call on their line. The incoming caller is just as surprised as our staff member.

How can an incoming call bypass our IVR and get picked up by any extension when a person lifts their handset to their ear?

Thanks in advance,

This sounds like ‘glare’, are you using analog trunks?