The phone is ringing. It can be Jack.
As far as you know, Jack is physically able to call you. It could also be anyone else able to dial a phone. No signal of likelihood, simply possibility.
The phone is ringing. It could be Jack.
Jack is not only able to physically call you, but you are expressing at least a slight likelihood of it being him.
The phone is ringing. It might be Jack.
This is sort of the opposite of the first case. You do not know if Jack is physically able to call you or not. But you think it likely that he call you around this time.
The difference in connotation between could and might probably varies as much with vocal intonation as dictionary meaning. Perhaps there are also regional variations. Let's compare three first-person cases to minimize unknown factors.
I can do that.
I can do that!
I have the physical ability to do that. As before, neutral on likelihood. The exclamation mark adds a degree of realization.
I could do that.
I could do that!
This becomes heavily context variant, and if spoken, intonation dependent. With the period, and no other context, it is almost as if you are silently adding, but I won't. With the exclamation mark, it again becomes a positive realization. I didn't think I could, but I can!
I might do that.
I might do that!
Again this would be context variant. An exclamation mark doesn't seem to change actual meaning that much. In either case you believe yourself to be (or will be) physically capable with some degree of likelihood. Stressing might in a certain way, though, can reverse it with an unstated, but I might not.