I have a Motorola Moto G3 which connects every time with my Columbus, and works really well.... until it (half) goes to sleep.  The display for the signal bars remain, showing it's connected, but the bars are empty, even though there's a good signal. 

 

Even in this snoozing state, if I make a call, it works just fine, though.  The problem is that when I receive a call, I miss it because it goes straight to voicemail.  I have both the phone and the car set to their maximum awake times, but there isn't an option on either to stay awake permanently (assuming this is the problem). 

 

I've turned on the developer options on the phone which allows a "stay awake whilst charging" option, but I don't know yet if this works, and of course, it means plugging in the phone for every journey.  I'm not sure if it's the phone or the car that's dozing, though I assume it's the phone.  I've been through every sub-menu on both phone and car, but I can't fix it.  Does anyone have a fix for this irritating problem?

At a guess I would look at the method of connection your phone and Bluetooth are working on as it's possible the issue lies there. Different Bluetooth modules can use different connection protocols such as RSAP (remote sim access profile) and HFP (hands free profile) for instance. It might be that connecting in the wrong way which means they won't be communicating properly. It's only a guess, and I don't know anything more that I just said, but I'd be looking at this perhaps.

I always use the handsfree option , that one doesn't take over your sim