If it possible for yall to video your husband reading some of your sons favorite books to them and then play it at bedtime. Video the boys playing or talking to daddy, showing them what they did that day, what they may have colored or created or something new they learned like how to do a somersault or something. Secretly record your dinner conversation & send it to him. The reason for the secret is just so he feels it's a normal day at home, not something put on, ya know? Another thing you can do with books is to get 2 exact copies of the same book. Then have your husband do an audio recording of him reading the book with BEEPS when it's time to turn the page, like a read-along book you buy.
Phone communication with children this age is very difficult as they don't understand the concept that clearly. Heck I still have issues with my 9yods drifting off in his phone conversations. If your husband is in more than one place, like traveling state to state or something, have you heard of Flat Stanley? http://flatstanley.enoreo.on.ca/ It's a very cool project and you could adapt it to suit your needs. If the kids have a stuffed animal they like, that they can do without, your husband can take that animal to the different places he goes for an adventure (if he can do that). He could take pictures of him with the animal and then write about what they did there. You, in the meantime, could get a map & put pushpins in all the places the animal got to do that day!
My husband is former Navy and we were always thinking of ways to make it feel as if he were still home. Unfortunately, he served on a Submarine and couldn't do near all those things, we only got to exchange letters. But many of the wives I knew had husbands on the ships & they could do those things. Oh another thing, is if you can afford it & can do it where he is, is to get cameras on your computers & set up a time to do a video conference thing.
Have your boys tell you one or two sentences of what they liked about today and write it down in a diary of sorts. Then when you can, mail it to him. It might seem small and insignificant to you but it will mean normalcy to your husband. I would have my husband do that on the sub. Just jot down a daily record of things you did, that you can share of course, and when they have 'mail call' rip out the pages & send it to us. It was so much better than getting a rushed 1-paragraph thing from him (mail call wasn't always scheduled on subs).
I hope those are some ideas you can work into your day.
One more thing. IF you can afford it, go by yourself and pick up some little things for X amount of weeks he'll be gone. Then wrap them up, hide them, and each Friday pick one out for them to open 'from Dad'. It doesn't have to be expensive or huge, for instance, you could do a collection of farm animals for the 2yo. Start him with a 'farm' then each week he gets a new animal to go in it. The 4yo, if he's like my sons, LOVE cars. So a race track to start, no electric of course, and then each week a new car to put on it. Or stickers, bubbles, balls, stacking cups, etc.
I hope yall are together as a family soon and in the meantime, I hope some of these can help pass the time. And if you have a picture of your husband in a frame, you can set it on the table at dinner time. Little ones, try as they might, sometimes are very skiddish when dad comes home from being gone so long. But if he was 'there' for meals it might help.
Good luck.