A user has reported that during a phone call to the user's SO (significant other), they both heard one or two beeps followed by a voice (English) announcing something similar to "This phone call is now being recorded". The call continued afterwards.
Both were using iPhone 7's on a Danish carrier/network and do not use Google Voice (pressing '4' can prompt recording of the call there with a similar announcement being made). Neither the carrier or the network provides such a service as far as we know. The iPhone 7 doesn't have a native call recorder and they have not installed one themselves.
I'm looking for (serious, realistic) suggestions on what might have happened here. What can cause such an announcement to be made?
The user was a bit confused and scared, so I may update minor details later, when the user have had time to talk with their SO to detail the event further.