My ex never went to rehab, but we did a temporary separation. One thing I regret is fixating so much on him and what he was supposedly doing for his fake recovery. I wish I had spent more time thinking about my boundaries and untangling the enmeshment I had with him. I used to ask him about his "meetings" (that he wasn't going to) and his therapy (the same therapist he'd been seeing for 5 years without any real results).
I went back to the same nightmare to give him another chance only to find that his whole "recovery" was a lie and that he'd been drinking the whole time. He'd even recruited his mom to help him cover up a public intox arrest that happened the day before we returned. She flat-out lied to my face about that, actually asked if I was sure it had really happened (I saw the arrest sheet because he was waving it around and ranting about paying the fine). She only recently came clean that she'd not only known, but had picked him up from the police station (it happened almost 3 years ago).
If he wants to talk to the kids and they want to talk to him a few minutes a day, then maybe that's what the phone calls could be. I don't think there's anything to be gained by having big discussions about his program or your relationship right now. I agree that he will probably be working overtime to convince you to let him back home right after rehab. Right now he uses you as an emotional garbage can and you accept that. A few days without drugs won't cure him of being manipulative and emotionally abusing you for personal gain.
In another thread you were hoping that this stint in treatment would result in limited or no contact for awhile. You can still set that boundary. If he gets mad, then he is in the right place to get the emotional support he needs to deal with it. I know you're afraid of setting him off and being the bad guy (ask me how I know), but that's part of the problem. You're afraid of your husband. That shouldn't be the case.
Hawkeye was right that there are bigger problems here than just drugs. The firearms and violence are not going to be addressed at rehab. That's a separate issue and won't simply go away because he has 30 days sober in a controlled environment.