It's now so easy to keep in daily contact with people that many people will if they can. Are you a person who keeps in very regular contact with your friends? Do you value the friends you talk to more often more than the ones you only talk to once in awhile because of having busy lives?
So this isn't the same situation, but I think similar enough. I have a partner who I only talk to once a week, as I prefer to be in more frequent contact with good friends or people I date, it was stressing me out sometimes. I have made the effort to make a few more friends lately, and have found that by being busy talking to and hanging out with them that I am feeling more happy in general and not even noticing not getting what I'd prefer out of communication with the one partner. Do you have a good support system of friends? Hobbies to keep you busy? Interesting people you want to spend time with on a regular basis? If not, find more.
Really, back in the day when I played a MMORPG, there was a chat room with a bunch of people in it and I'd go in there almost every day to chat with the people there - because they were entertaining, because I liked some of them, because I felt they were friends that added something to my life. Not because my interaction with them was better or more important than real life local friends. Not sure if you might want to ask your bf to not talk about his other friends as much when you're talking to him? That is not a horrible thing to do. Do you worry he values NON sexual friends he speaks with daily more than he does you?
So uh..why do you know how often he is talking to his friends, or how much emotion he puts behind his telephone greetings? I think it's time to just stop being involved in that stuff for awhile (don't ask him about it, and ask him not to bring them up for a few weeks), until you get your feelings better sorted out. Could it be a case where you aren't jealous so much of the FWB but you're not really tolerating caring about somebody so much who is not local to you. Is the FWB a recent addition? (Since you say 4 weeks you've been stressed out)
Another plug here for If the Buddha Dated
. I had more fears similar to yours back when I was in a longer distant relationship with my current husband, and I found it very helpful for feelings I had then that were a bit closer to what you are describing..