Hi, I have two iPhones, a 6S and a 7, which are both running the latest version of iOS. The 6S is my business phone and the 7 my personal phone. Normally I have my personal phone paired with my in-car audio system to listen to music, but the 7 doesn't play nice with my car's bluetooth. It takes forever to connect and often spends a minute dropping in and out before it settles down. When I connect the 6S I get a little connection strength display on my car stereo and it works flawlessly. 

The problem is that I cannot connect both phones to the car at once - it only allows pairing with one device. So whichever phone I connect means I cannot take handsfree calls on the other. I have call handoff set up on both devices but it only works while I'm at home on the wireless network. As soon as the WiFi is off, handoff stops working. 

Both phones are on a Skinny pre-paid connection. I note that Skinny doesn't support call forwarding, but I'm not sure if that's the problem here. My sister has two iPhones (a 6 and a 6S) that she can use handoff on even without WiFi and as far as I can tell, all my settings are identical to hers. The car is a 2010 NZ-new Toyota Yaris with the factory-fitted headset (although I've added an amp and decent speakers).

Any idea what I'm doing wrong? So far Google has not produced a solution, and I would dearly love to be able to have both phones work handsfree in the car.