I'm in much the same situation, Tamara.
1. Keep calling on a regular basis and make a log of each time you call and if you got to speak with the kids or not and if you did, for how long. Log anything that was said by your ex as well.
2. Writing letters is good too. If you can't call, do write and do let your ex know and the kids, why you are unable to call and encourage letter contact.
3. Absolutely get in touch with an attorney in your state in the US. There's a few ways to do this. I recommend checking with your state's bar association, which should be easily found on the web and then call them for a referral. This is probably the best way to ensure you are getting someone reputable unless you have a solid word of mouth recommendation or you already know of a lawyer you would like to use.
I understand about money being tight and such. Unfortunately, with no solid agreement in place, you are at your ex's mercy. I will say that he cannot go to Court and take legal action unless the Court is satisfied that you have either been notified or attempts have been made to serve you with papers to no avail. Theoretically, even if he did and had some kind of order in place, you could still go to Court and there's no Court in the US that is going to say that a non-custodial parent doesn't have the right to some type of visitation. Unless your ex has some very damning evidence that any kind of contact between you and the children would be harmful to them (HIGHLY unlikely), then it's just not going to fly. There's no way he can keep these kids from you and not get legal retribution for it.
SO, with all that said, find an attorney. Get something in place and protect YOUR rights. The idea of shelling out the money is daunting. Most attorneys will take payments. Borrow if you have to. There's no way those children should be kept from you.