This has been covered in a few other threads, but I will repeat a solution that worked for me:
Streamer Pro 1.3 had been dropping calls from iPhone 5s and iPhone 7 ever since moving beyond iOS 9.2. I have more than one Streamer, same results. I had both updated to latest firmware (3.5.3) and latest iOS 10.2 on iPhone 7.
1) Delete all pairings from iPhone Bluetooth settings.
2) Delete all parings from Streamer Pro by holding 'volume up' and 'power' simultaneously for 3 seconds.
Now re-pair Streamer Pro by outting it in pair mode and then re-pairing from iPhone 7. This has eliminated the call drops for me.