I have also been having these "disconnection" problems with bluetooth connecting my iPhone SE to my 2006 Lexus RX400h since I upgraded to IOS 11.x. Apple contends that the problem is that the Lexus has an older BT spec than the iPhone. They say that the issue can only be resolved by upgrading the car BT firmware. Lexus knows of no upgrade that is available. I have discovered a work around for my issue and possibly a clue for Apple.
My symptom is:
I start the car and shortly after that the car's display console says “Bluetooth Connected.” After a short period of time this connection seems to go to sleep. If I make a phone call, the iPhone initially displays the Audio choices as follows:
Bluetooth Hands free ( My car )
iPhone ( Hands on the phone )
Speaker
These three only display for about two seconds and then the “Bluetooth Hands Free” option goes away. It switches to “iPhone” and “Speaker” only as choices. iPhone will be selected, so of coarse I pick up the phone and it is ringing on the iPhone. About five seconds after the caller answers and the phone call is connected, the iPhone audio goes dead and in about five more seconds the audio is switched over to “Bluetooth Hands Free” in my car. Of coarse I have missed most of what the other person has said in those five seconds or so and they are usually saying “Hello…. Are you there.” Aggravating….
The work around:
This has worked every time so far.
Before I make a phone call I go into the settings and open Bluetooth. It shows the connection to the car ( Bluetooth Hands Free ) as “Connected.” I tap that connection and the little pinwheel spins, then goes back to “Connected.”
I can now make a call and the audio is coming thru my Lexus Bluetooth connection.
This has worked every time for two weeks of doing this routine. I believe that the iPhone's bluetooth connection to my car may be going to sleep to save power or some such thing. Reselecting the connection on the iPhone, in settings, works to eliminate the dropped connection and dead air time in the beginning of the call.
I hope someone from Apple reads this and it helps to solve the problem.