Single Beep before calls

I purchased an iPhone 7 a few months back and recently when making outgoing phone calls to mobile numbers it now makes a single beep tone before the ringing starts. The phone call connects and there are no issues with the call. I am not getting messages/ notifications or any incoming calls when the beep happens.

Just wondering what it means?

I have owned multiple iPhones previously and never had this issue, I also owned the phone for a couple months before this started happening. I have not changed any settings.

Tara