The official Apple technical support forum is actively discussing the problem of connecting the iPhone 12 and iPhone 13 via Bluetooth to car systems. Users note that the connection is unstable, and incoming calls hang or drop. Updating to the latest version of iOS does not help resolve the issue. Judging by the reviews, it is present in both iOS 15 and iOS 15.1.1.
Most often, problems arise with Toyota car owners. However, on the forum you can find several complaints from drivers of other brands. In particular, owners of Audi and Volvo report incorrect hands-free operation. However, there are few such complaints.
There is an assumption that in the first place the problem affects cars without CarPlay support. However, this is only speculation so far. How to restore hands-free work is also unknown now. It can be assumed that in the near future Apple will release a small iOS update that will fix the situation.
A source: 9to5mac