Just FYI - I am in documentation phase for my spouse's visa. Yet to apply. We have been dating for few years from 2 different countries. We do not have much chat details, because we use facetime a lot. As you are aware, phone company wouldnt provide the bills for internet calls ( which shows the number of caller, whom the call is directed to, and minutes. I was able to recover most of this information with "smartphone recover pro" app. The output is in CSV form. I am also attaching a letter stating this to visa officer.