Welcome back to JustAnswer. He is probably cold because it makes it easier not to want to go back to you or she may have been close by when you called him also he could have been upset that you were calling him because he doesn't want the children with the ex wife to know he continues to talk to you. If he says he loves you then he means it but realizes that he has to love you from afar right now because of his children. Ask him why he does not call you even when he is at work or out running errands but I'm pretty sure the answer will be that he wants to make this work for his children and if they find out he still talks to you in any way they will be angry and hurt, he made a promise to his children that he will come back and give it another try but if he doesn't love his wife then he will only be half trying because he wants to be there for the children but the children when the family (including is ex) back together.
He is living a lie and usually a lie comes out in the open because they usually find themselves covering a lie with another lie. Even though it's clear that he is there for all of the wrong reasons he has to realize this and want better out of life. It's now time for you to decide if you want to continue this way or if you think it's time to make some decisions about the way you want to live you life. He is treating you as his mistress and he isn't even married anymore. Do you want someone in your life that you have to sneak around with and he doesn't call you, you have to call him and when you do he doesn't act happy to here from you until the end of the conversation?
He wants you to hang on and wait for him and the only thing about that is you were giving a time frame as to how long you will have to wait. he is making himself live in a living hell just so that the children don't suffer but with prolonging leaving he is hurting them so much more than he ever realizes because he isn't there for the right reasons and when the time comes that he truly wants to leave he won't be able to because the children will tell him that they want him to stay and then there he is again making the ultimate sacrifice and that is his happiness.
You're exactly right about him neglecting his feelings for the sake of the children because neither of the parents are happy with each other. Only your boyfriend knows the time frame to which he is will to be unhappy. Yes there is definitely hope for the future but he has to be willing to see that this arrangement is doing no one any good. It's adding more stress to everyone involved especially when the children know their mother was happier without him there. So they know that both parents are happier without each other but yet they still want their father to stay there even though he is not happy that is very selfish reasons to keep him there but it's up to the parents to be the adult and tell the children they no longer want to live that lie and deserve to be happy even if that means them finding happiness with other people, until then all you can do is wait and hope.
Yes he wants to make sure that his children don't feel like he deserted them like he felt when he was a child and he will do anything (including give up his own happiness) so that they will know that he is there for them. Him choosing happiness is not necessarily deserting them but he has to be the one to realize this. His being Mormon is only partly the reason for him being so loyal to his family but he has to realize he is divorced and got divorced for a reason. I would suggest counseling for him to deal with his past and that may help him to see that what he is doing though it's noble but it not good for anyone and a counselor may be able to help him see that.