I don’t contain a full complete help, but partial.
Under Android latest versions you have to select for calls under the Bluetooth audio pairing.
This allows for buds to be paired and connected to two devices at once, one for media and calls, the other for only media only. So a phone+tablet example where you can listen to media from tablet but a call comes to phone. This overcomes many buds limitations of not able to connect to two devices at the same time.
Android wont let you do this for everything. Concentrate on all of your pairings, good idea to delete all pairings and only pair to one phone and one non-phone (tablet).
Screenshot off my OnePlus, see the phone calls selected.
It’s not always enabled by default.
On my OnePlus phone I have this phone calls enabled, on my tablet its not enabled.