Yes, phone sex IS cheating!
This ought to be obvious from the actual subject. While “Phone sex” does not involve direct physical contact, it certainly crosses the line emotionally as far as “forsaking all others” is concerned.
But among you there must not be even a hint of sexual immorality, or of any kind of impurity, or of greed, because these are improper for God’s holy people.
-Ephesians 5:3, NIV
I would suggest that “phone sex” is definitely more than “a hint of sexual immorality.” It is improper behavior for a spouse to do this with anyone who isn’t their covenant partner.
Again, I would not underplay how problematic this behavior is!
It is stolen intimacy. And it is a violation of the marriage vows. It needs to be taken as seriously as physical adultery, in my opinion!
Christians need to stop being naive about how damaging this sort of behavior is! They need to stop giving such behavior a pass. We are instructed not to tolerate even a hint.
Sadly, too many Christians are fine with tolerating more than a hint of sexual immorality as long as it “wasn’t physical.” That’s not the biblical standard, though.