I have a Panasonic KX-TG7875 phone system with Bluetooth connectivity capability. An iPhone 5 had been set up and worked fine, it would automatically reconnect when the phone was in range. Changed to an iPhone 6s Plus and was able to connect manually but it would not automatically reconnect.
I followed the suggestions from brucecam
1. unpair the old cellphone (MENU #6111 on my system, may be different on your system)
2. unplug/replug the power to the phone base
3. pair the new cellphone (MENU #6241 on my system, may be different on your system)
After this the iPhone 6s Plus would reconnect automatically when the phone was back in range. I had tried just the unpairing/repairing steps and it didn't reconnect. The power down seemed to make the difference.
Thanks for the tip.