I'm sorry for all the annoyance this is causing you, I hate that feeling, esp. near the holidays!
I saw that some poster on your other thread was getting bashed, but I agree with her. I can't imagine being spoken to the way BM always texts you, thus giving her a dose of her own medicine must feel really good! However, I'm not sure it does anything but create a vicious cycle: she orders you around, now you are ordering her around. Next time she does this and you get upset, you don't have a foot to stand on b/c you are doing the same thing!
Not married so thus I am not a stepmom, but to keep with acronyms, BM does this all the time to boyfriend: he gets a last minute order to change things. We generally do have plans when we don't have our kids, so this genuinely messes things up for us. However, I admire him for going with the flow b/c that's what's best for SS, and if we have plans we can't break, he just says Sorry, No. I think the worst thing he could do is start being as inconsiderate to her as she is to him, I would be unable to respect either party. (Admittedly, BM and I don't text directly, so I have never been experienced being the subject of her demands.)
Back to this post: I guess saying something like 'Can you talk to her or have your DH talk to her about her tone with you/unreasonable demands' is out of the Q b/c that seems like the most logical response, but I consider you a logical person and you aren't mentioning that. I must say, I'm not sure how she could have worded this request (for the phone) more nicely. She didn't demand by saying 'You need to return lego and get him a phone instead", she asked. Am I missing something (other than your history with her, I get how that weighs into this)?
If anything, this might show how giving her a dose of her own medicine doesn't really solve anything or make you feel any better.
I say all this at the risk of your wrath- I hope you know this comes from a place of respect- I appreciate your responses to my (and others') problems, and I want to understand more.