Definately sounds like a communication breakdown.

Your DP should have consulted you before giving the ok. He should of then officially told them that you wouldnt be able to go. Then imo it would be up to those organising the night away to keep everyone up to date about the plans and what not.
I understand that you and your DP were under the impression that they werent expecting you to go, especially if no further plans had been made with you both.

Sounds like you and your dp are in need of a break from them. IMO I think both sides are in the wrong. I mean they were planning an occasion and should have kept you up to date and just not assumed that you knew what was going on. But also you or DP should have asked them. (Although I know myself, that regardless of how good a friends we are with a couple, if they ask DH something I will not ask them something about it, unless they bring the topic up)

As for not answering your phone, well I understand that. I refuse to answer my phone if something else is going on. I wont answer it during meal times, bath times, or if the girls are having tantrums. I've told everyone that if I dont answer and its an emergency than leave a message to call them back asap.