This would definitely be a phone setting, not anything you could modify on the Roam itself.  The Roam is just playing the audio the phone is sending to it. Someone here might know the answer, but you are better off talking to android/iOS support.

And I suspect is slightly difficult to do, since when you’re connecting to the Roam, you’ve said “send all output from my phone’s audio system to this device via BlueTooth. I can’t think of any settings in iOS, which I’m more familiar with, to say “send this music data, but not other OS data”.  Not very familiar with Android, but it’s hard to imagine it’s much different. Either you’re sending everything, or nothing, via Bluetooth. 

On android, you can set the allowed service for each bluetooth device you can connect to.  For example, you can turn on/off for calls, audio, input device, etc.  So headphones could be on for audio and off for calls for example.  But the Roam can’t be turned on for audio.  It doesn’t play call at all.

I think what your wanting is to be able to take a call on your phone with whatever music is playing (on a bluetooth speaker) stopping during the call.  I don’t know that phones can do that as they aren’t really designed for that sort of multitasking.