Dealing with an absent parent

I'm have been a step mom to a 5 and 6 year old for the past 4 years. There mom left when they were 1 and 6 months. My husband and I have full custody of both of them. We moved to a different state after my husband finished his 5 year contract with the Army. The kids mom hasn't really tried to maintain contact with them to keep a good relationship. The kids really don't know who she is. We tell them who she is and if they ask a question we answer honestly. We don't ever bad talk her or shame her in front of them. She hasn't seen the kids in two years and blames everyone else. She goes weeks without asking how they are and all she does is send a text. We told her she needed to step up more because the kids are noticing a lot more and have a lot more emotions. She doesn't work and doesn't pay child support because we know we won't get it. She hasn't sent a birthday or Christmas card in the past 4 years. We don't really know how to handle it anymore. The kids when she does ask to call once every other month don't want to take the phone to talk. We don't want to force them, but what can we do make sure our kids are emotionally okay?