My step-son lives with us during the school year and his Mom is across the country. Their favorite forms of contact are all electronic, so it sounds like those might not work as well in your family. Nevertheless, I will mention them, in case you'd consider one or two; or modified versions of them.
* DSS has his own cell phone. If your ex has an iPhone, does that mean he has AT&T service? If so, he can add a second cell phone for an additional $10/month. Your kids would share whatever minutes your ex buys every month, so unless they spent a LOT of time on the phone with you, your ex would not have to upgrade to a more expensive monthly plan. A variety of phones are offered free, when you add another line and commit to a 2-year contract. However, if you're concerned about limiting use of the phone, you might think about a Firefly. I don't think they make them anymore, so you'd have to buy one on eBay. (Then just walk into any AT&T store and they'll give you a new Sim-card to link the phone with the second line your ex adds to his plan...) Parents go online to program #s into the Firefly address book and kids can only take/receive calls from those #s. There's no keypad on the phone, just buttons for Mom, Dad, 9-1-1 and arrows to scroll through the address book. They're really awesome! DSS rarely spends more than 10 minutes on the phone with his Mom, but there's something valuable about hearing each other's voices, even if it's brief. Plus, kids think it's really neat to have their own phone. PLUS Dad's iPhone stays safe!
* We ALSO pay for unlimited texting. I think it's $10/month, for all the phones on our AT&T plan. (You can't text with a Firefly, though.) Texting may sound like the purview of teenagers, but DSS has loved it since he was pretty little. Kids figure out how to do it quickly. This may sound awful, but it's a really nice way to let each other know you're thinking about each other, without the commitment of a phone call. Sometimes kids will hesitate to make (or take) a call, because they're in the middle of something and don't want to stop. However, it only takes a few seconds to read or send a text. Often, kids have trouble during a phone call, remembering what they did/thought/felt in the hours or days before that call, making calls unsatisfying to a parent who misses them terribly and would love to hear details. Texting gives you tidbits about what your child is doing/thinking/feeling right this minute, since they don't have to stop and focus on a conversation.
* Facebook may also sound like it's not for kids, but I think it's WONDERFUL. You can send private messages to each child OR post things that all of you can comment on (kind of an extended group conversation), plus post photos, videos and links to interesting articles or pictures on the web (sort of like looking through an album - or listening to a story - together). There are ample privacy settings that parents can control, and your kids don't necessarily have to spend a lot of time on it, keeping up with everyone they know. Perhaps, for now, FB could be just for you and them. DSS is definitely more interested in checking FB than he is, checking his email - and much more inclined to be responsive on FB. I think that's because there's no pressure to compose a long letter - short and sweet is just fine, on FB! FB is actually my main way of staying in contact with DSS, when he visits his Mom in the summers. I miss him, but his Mom is pretty hostile toward me, so he feels awkward about talking to me on the phone, when he's around her.
* I'm sure others have mentioned Skype. Unfortunately, DSS's Mom refuses to do that (so I can't give any personal info. about it), but it sounds like an ideal way for kids to keep in touch with an out-of-town parent. If you're concerned about too much time in front of a screen, only do it once a week.
DSS LOVES getting things in the mail. Cards, but especially any sort of gift - even little, silly things like a souvenir pen or pad of paper from some place his Mom visited. She used to send him something every week. I think that would be a terrific habit for you to start. Just don't get hung up on getting mail in return. DSS hates to write cards/letters; and he also hates for people to give him a hard time about it, "I'm going to stop communicating with you, if you don't respond to each of my letters..." If your kids write back, great! But if they don't, just rest assured they're enjoying receiving what you send; and that it shows them that you think of them.
I think photos are also essential. DSS's Mom sends a photo album, after each of his visits with her. I'm sure sometimes he feels like he never gets to spend time with her and everything stinks, and those remind him that the two of them do have enough time for some good fun together. We also send him an album (kind of a yearbook), when he leaves for the summers, to remind him that there's a happy family, and friends, waiting for him back here, when he has to say goodbye to his Mom for another school year. I've seen him looking through those albums here, so I know they mean something to him.