M.:
Chatting with previous boy/girl friends is okay in my book. Talking sex with them - nope - not unless we've okay'ed from the beginning.
Having a phone that is NOT in your plan, a pre-paid, throw away? NOT ONLY NO BUT HECK NO!!!
Communication is key. Yelling and screaming at him won't change it or his behavior. You BOTH need to go to counseling to learn how to communicate with each other and work through this together. It could be that this girl actually tripped his "go" switch and brought on some serious sexual side affects - it's been known to happen.
DO NOT be condescending, you are NOT his mother - you are his wife - HIS PARTNER - while I agree that you should NOT be intimate with him - you MUST communicate with him...do NOT withhold communication as it will NOT help the situation.
If you are going to do anything - DO IT - don't do it half-way. He made his bed, now let him sleep in it. Your daughter doesn't understand what's going on - you just need to let her know that daddy loves her and that you guys are having adult problems....use the KISS method (Keep It Simple Silly) and don't over do it with her.
I wish you Godspeed and the best of luck!