Sorry to hear you're in this situation. It's a tough one.
I'd encourage you to start by changing the way you're thinking about this. You're asking us how to convince him he's being unreasonable. From his perspective, he's not, and as an unbiased third party, I don't think "I don't want to have phone sex" is all that unreasonable a thing to say. What you're really asking is, "how do I convince him to give me what I want?" I know it's uncomfortable to acknowledge that you're asking for something selfish, and ignoring his needs, but it becomes a lot easier to diagnose and address the problem when you recognize this for what it is.
I may be wrong, but it sounds like your husband is not enjoying the phone sex. If you obligate him to continue, he's only going to get more resentful and uncomfortable with anything sexual with you at all. Obligated sex tends not to be good sex. Instead, a better question could be, "what can I do to make this more fun for my husband?" Maybe your husband's a more visual guy, and doing a video chat over skype would turn him on. Maybe there isn't a solution short of getting physically together again. Exploring it from a perspective of "how can I make intimacy enjoyable for both of us" rather than "he's wrong, how can I get him to know that he's wrong so he does what I want," will probably make you both a lot happier.
His comment about masturbation making him feel "weak" is a cause for concern. I'd suggest asking him about that. "What did you mean when you said masturbating made you feel weak?" Be genuinely curious. Don't just ask about it to try to manipulate him into phone sex. Maybe masturbating makes him feel like "less of a man." Maybe he finds it sapping his motivation. Maybe it was purely an excuse because he's not enjoying the phone sex. The only way to find out is to ask him, and be truly interested in his response.
Remember, at the core of any good marriage is the friendship. Take an interest in his needs and desires. If you don't, you're both doomed.