I am an extremely possessive and jealous person who is trying to tone it down as my jealousy has spoilt many occassions for us and my boyfriend has says it makes him feel like shit that I think so lowly of him.
I could barely read this and all the posts in response, but you know what i believe your husband. I just don't think they'd hand over the phone so casually and that he wouldn't immediately sound nervous on the phone and start making excuses. So I agree with funkyderek.
However in your position I would be shaking, hysterical and furiously angry. I would like to think my boyfriend would be way to afraid of the consequences of doing anything extracurricular. I dumped my last boyfriend of 3 years just because he had kissed somebody - I knew he had even though he denied it. I think the way you can tell is that a man cannot help grinning a little like a schoolboy when confronted - its a nervous tic and not actually restricted to men as I have found myself at it when I've been foolish enough to lie about my own misdemeanours. Its a horrible grin and probably the most unpleasant look on another human being's face - well to me anyway, which is why I resolved never to lie like that again and better still never to be unfaithful again.
Next weekend my boyfriend has a work do and is staying at a hotel afterwards even though its only 20 minutes home in a taxi - I have been very unhappy about this and made it clear since I found out about it and it was causing terrible arguments. So now I have gone on the charm offensive - I don't think he wants to stay over so he can cheat with the only female who works in his office, but you can't help that niggle.
Like you I am afraid that the whole intense JW thing of no platonic relationships with the opp sex and chaperones for the most innocent things like my Dad taking an 80 year old sister home from a meeting requiring me to babysit them has warped us a little. We were taught jealousy from the very beginning so we can;t help being a bit more than normal. The way I see it is that I would rather be a little jealous than complacent and smug and the last to know if something is going on with my partner. To that end I do take measures discreetly and make it my business to know and get on with all of my boyfriends work mates and friends, never unless I absolutely have no choice throw up going out on a night out with him when I am invited. I also admit to checking all receipts although this isnt through jealousy 100% - I have to do it to make sure he keeps all the ones relating to work expenses which are frequent, and I do check his phone in front of him. I don't do it as if I'm checking I just have a quick look whilst popping it on to charge for him or to delete some old texts of mine to make room for new ones. So do what you can to check on him, and I would say unless the mutual friend you have spoken to has her own personal agenda then the best testament is what other people - your friends say about his general behaviour when you aren't there.
Trust your gut instinct, but gut instinct isn;t the same as irrational fears - so learn to discern between the two and when you can't do a little private detective work of your own. Its not failsafe and I am not for one moment saying your suspicions in this case are irrational. I would be initially ballistic, as I was when I read this but then I read again and I do think whilst he has been very unwise indeed (and I would say you were unwise too, even though properly motivated, to encourage him go if you know there would just be 4 people 2 of each gender) I think he sounds innocent. You'll know when you see him whether he was true to you or not, unless he is well practiced at this.
Don't panic, be loving and it is possible to sleep in the same room with the opposite sex and not do anything because I have done dozens of times - maybe I'm just not that sexy! EEEK
Hugs and let us know how things are when he gets home!