Why did this happen?

Technically, it could be any number of reasons: a new medication, close to that time of the month, a late reaction to something upsetting (that happens to me a lot. Something distressing will happen but it won't bother me for a week or two. And then, out of no where, I'll start crying), whacked out hormones.

If you're feeling less attached to your boyfriend (and you think this is the cause of the crying) then perhaps you should take a break from the phone sex. Try doing things together; going to a movie, or out to dinner (or something like that). Try and bring back that emotional connection.

Also, maybe it's time to look at your relationship and decide how you really feel about him. Perhaps your feeling emotionally distant because those feelings just aren't there like they used to be.

Maybe that could be it, you desire more connection from your boyfriend. Phone sex isn't really something you can bond/connect on in my opinion. Maybe you can talk to him on how you feel about this and how you feel about him emotionally. You two should talk on that and help strengthen your relationship. I hope that helps. :/