I am copying my posts and updates from another thread that is about the same problem.
iPhone 7 Bluetooth constantly disconnects and reconnects from my Ford SYNC system.
https://discussions.apple.com/thread/7670674?start=0&tstart=0
01-05-2017
I have the exact same problem with my iPhone 7 and my 2014 Ford F-150 with SYNC. It worked flawlessly for almost 2 years with an iPhone 5S. As soon as I upgraded to the iPhone7 it started randomly disconnecting and re-connecting the Bluetooth connection during every phone call and sometimes while playing music. I tried deleting and repairing, restarting the phone and using another iPhone7... nothing has fixed the problem. I contacted Apple And theye opened a case, but after troubleshooting they said I need to have the firmware in my vehicle updated. I just looked it up on MyFord site, and I have the latest firmware, there isn't an upgrade.
I think this is an iPhone7 issue as I can use any other version of iPhone (other than 7) and it works perfectly. I am disappointed in Apple's inability to recognize and solve this issue. I have loved every Apple product I have owned, until now...
Any feedback from others who have solved this issue would be greatly appreciated.
01-16-2017
Update to my previous post:
I checked the My Ford site again, and confirmed that the iPhone 7 is not listed on the SYNC compatible device list for SYNC version 3.7. I tech chatted with Ford SYNC tech support to advise them and they told me that there is actually an updated version of SYNC (v 3.8) for my vehicle, and it must be installed by the dealer. The My Ford site says that my vehicle has the most updated version and there isn't a version 3.8 listed, but the Ford SYNC tech support rep was confident that this upgrade will correct the problem. I have a service appointment this week and will update this post after I get the upgrade to let everyone know if it works or not.
01-24-2017
Latest Update: I had my Ford F-150 at the dealer all day today. They installed the latest version of Sync on my system (SYNC v 3.10), and did a hard system reset. Unfortunately, my iPhone 7 still does the same thing: while using the phone or voice texting through the SYNC system, the Bluetooth connection will disconnect and reconnect randomly. Sometimes it will go several minutes without disconnecting and sometimes it will do it every 20 seconds. It never drops the call, it just disconnects and reconnects the Bluetooth connection. This never happened with my last iPhone (5s), it worked flawlessly for almost two years. I am going to update my Apple service request, as it appears the ball is back in Apple's court.
01-24-2017
Second call with Apple support. They had me reset the phone to factory settings and set it up as new. I did, it didn't solve the Bluetooth connectivity problem.
01-25-2017
I had my third call with Apple support, my senior advisor scheduled an appointment for me to bring the phone to the Apple Store for further testing.