I have a theory on the "I don't know it just happens." part of this issue, which your description seems to indicate he does seems to be attracting them and that it is genuinely platonic. Another guy on the thread does too. And that does, actually, happen. And actually, there's the interest thing (pottery guy) but if you have a circumstances like this where no matter what, it seems to keep happening, sometimes there's a more psychosocial reason.
There is a limit to the way straight men are allowed to interact with each other emotionally in the West. It hasn't always been that way but that is how it is now. Get too soft, too emotional, too vulnerable, too close, and you could be considered "feminine" or "gay" - the ultimate transgressions against traditional masculinity. The incomparable Tony Porter founder and CEO of A Call To Men, an sexual violence education and prevention organization focuses on healthy manhood specifically dealing with countering negative outcomes in the current socialization of boys in terms of respect, emotionality, understanding and understanding of the culture, deconstructs how the threat of being "girly" is used as a weapon against young men and how it represses their ability to connect with each other in his talks very simply. They aren't safe with each other not because there's something wrong with boys. There's not. The lesson's they're taught though, "Boys don't cry" "Be a man" "Suck it up" lead to a shared culture of emotional containment even when friendships develop. For a large number of men, friendships with not provide a safe or comfortable place for vulnerability and openness because of messages they received from family, friends, peers, older adults around them, the media, and society at large. Many of men who find themselves in those circumstances tend to rely on their romantic partner for all their emotional support because that's where it's "allowed" to feel things and share etc.
Now, of course, thats not the case of every man! Plenty of men are able to be vulnerable and connect with their friends, to lean on them for support and create deep bonds that go beyond social activities. But. The socialization is still there - the pressure of manhood behavior expectations exist even in close relationships, even if a given friend unit give that expectation the bird and share their souls to each other on the regular. That is just Western culture today. However, for the restrictions are different for women and so women aren't like that in their friendships. Their socialization is different. Friendships between women often foster emotionality and that open communication and support style. Why's that matter? Well, sometimes even men who are perfectly capable of being open, emotional, vulnerable and connected to themselves and other people 100% platonically because they've fully internalized that socialization is just that and they are emotional beings, find it easier to connect with women than men. Since women are just other human beings, why not? If it's not as for a man to connect with women, why shouldn't he create friendships with them? We all need friends and as a lady, you are probably aware that women kick ass. He seems to realize this too. Good for him.
SO! I said all of that to say this: there is a very distinct possibility that your boyfriend has a high number of female friends because he's an emotionally in touch human being who wants and values friends he can connect with emotionally but because of the way the West socializes its boys from the day they're born, has a harder time making meaningful friendships with men than with women. Which is shitty because it shouldn't be so hard for men to open up to each other but according to all my guy friends? It is very very hard but it's much easier with girls. And the fact that male/female relationships have been romanticized and eroticized as the most acceptable way for a man to have strong emotions, to the point where you fear he will accidentally fall in love with someone else? Only stresses the point.
If he has a lot of female friends, and you believe him when he tells you they are truly friends (And why wouldn't you believe him when he tells you they are friends? Has he ever lied to you before? Has he given you a reason not to trust him? It's been two years. You don't have to like them or know them but do you know him?) chances are you may have to face the fact that he is a man who is more comfortable with female friends. If so, the problem comes stems from him or his friends or even from with your jealousy itself but the structure of society as it currently functioning.
That in no way diminishes this problem at all or the pain you feel. What it does is changes what kind of solution your problem needs. If this is the reality of the source and type of friendships he has, then he's doing nothing wrong and you're doing nothing wrong - the world is wrong. But since you can't fix the world right now, it becomes about because if that is the source, then it becomes about acceptance and changing perspective and that is a much much harder thing to do than agreeing on phone calls and meeting times.
I think you were well on your way to the same conclusion. You came here asking to TALK about this issue, how to DEAL WITH YOUR JEALOUSY as a boundary, not tell him how to stop having friends. That's way more than I've seen people react to this sort of thing and considering the fact that you WERE yelling at him and now have moved forward to I statements and changing your own behavior, there is a very real possibility that you and your relationship are both having awful growing pains against the bounds of Western society's very small heteronormative box of what interpersonal relationships should look like between men and women. You are bigger than those norms are and I think you know that because you seem very aware that your jealousy-based anxieties don't make sense completely logical sense. Because of that, I hope you will remember that we girls also get awful messages that poured into our heads from the cradle - in particular about Other Women as predatory competition, man-stealers, and that only way for men and women to meaningfully interact is romantically and sexually we are flooded with by movies and tv and the internet inescapably.
I'm sorry I dont have any specific suggestion about how to go about fixing your circumstances. I hope you'll consider these more abstract aspects when you're facing this very concrete problem. I don't have the whole picture but if it applies, I hope it may help give you a new lens with which to examine your feelings, options and possibly your situation as a whole.