I would be very upset and hurt if my husband did that. there was a time in our relationship that he was secretly corresponding with his X. The worst thing about it was that he denied it, lied lied lied. Had he told me, I would not have been nearly as upset.
But phone sex, wow that is heavy. is he paying a service for this, or is he calling someone he knows?
If it is his credit card, his debt, and he is calling a 900 number or service,. then that to me, would not be nearly as bad as if he is having phone sex with a girl he knows in real life. If he is having phone sex with another girlfriend, and X, or a fuckbuddy I would be livid. And if he used MY credit card account to do this, he would be on the street sop fast his head would spin.
But if it was a service, and he used his own money I would just ask "what can we do to make things spicier, so that you won't feel the urge to spend all that money on a phone sex lady". Maybe he has a kinky side he is ashamed about. I would be gentle with him if that is the case. Good Luck. Love is hard. Relationships are harder...but worth it.