Taking the ex out of the equation, and ignoring whether or not your husband made the right choice, at the moment your stepdaughter is almost certainly feeling rejected. Her dad explaining to her why will sound like sermonising and excuses. She is 10 and she doesn't see things like an adult does.
If on top of that her mum is being negative about her dad, that will make reaching her that much harder.
My daughters' dad has never been bothered about seeing them much and they are now very negative about him, but recently things have started to improve.
What they wanted and needed at an earlier stage was to know their dad cared. Kids don't have the same love language as adults. For kids, time and attention is how you show love. If phone isn't working, try other ways to share time.
Watch a favourite film together at the same time over WhatsApp or Facetime or Skype.
Play an online game together.
Just turn up at the door to drop off a gift - 'I saw these and thought of you'.
What changed for my daughters was he started baking for them and dropping things off.
But he definitely shouldn't try to justify his reasons to her and I think he should reconsider the no proper contact.