I know gay couples you've been together for over 20 years, and the formula for success each couple has is quite varied. Some are strictly monogamous, while others have completely open relationships. Some permit a little playing on the side, so long as it doesn't come into their house or done within the other partner's sight, while some share 3-ways and group scenes right at home.
If a partner likes online porn or phone sex, my question would be: does it undermine and endanger the relationship? Does it represent a sexual lack, that his partner is failing to provide, and needs to address? Or does he just like that variety, on top of what he's happily getting & returning in their own bedroom?
The OP and his husband need to talk, as many here have already said. And work out some rules of engagement, as we'd say in the Army. Ya know, my own partner likes to visit Silver Daddies, where he has many friends, and I don't feel threatened. And I come to RJ, where I have many friends, some I've met in person and introduced to my husband, and he's likewise cool with that.
And if my hubby wanted to visit porn sites, or dial-a-sex, I wouldn't care so long as that made him happy. Only excessive cost might be an issue, because neither of us spends money selfishly or secretly on our own selves.
Our money is community property, belonging to neither of us individually but to both of us collectively. If we do squirrel a little away in secret it's only to save on surprise gifts for the other, never for indulging our own selves.
So I would accept a long-distance sex chat line charge for my husband if he asked me, knowing that was what he wanted. I'd ask him if I was failing him in some way, but if he said no, he just liked this other stuff, too, I'd be comfortable with it. I mean, can I cover ALL his sexual bases, nor he mine? His sexuality isn't defined or limited by what I myself like to do or can offer him.
What I wouldn't like is him going behind my back, as it happening with the OP. I don't dislike the action so much as the deception. THAT is the first problem that needs to be resolved here, in my view.