You should continue to not interfere. OTOH, your DH needs to let BM know that she has no right to interfere in what happens on his time and that, therefore, the schedule will be OFF until SD gets home and that, if BM persists, he will simply take his daughter's phone and return it when she leaves.

It's hard on kids who live FT with a controlling parent. SD likely knows that if she pisses off BM while with your DH, she has 28 days worth of wrath and vitriol she has to deal with from BM. Make no mistake, BM putting her in tears every night and trash-talking your DH is toxic. Toss in the control, and I'd probably consider it emotional abuse.

So, what should you do? You should do nothing. Your DH, however, has a choice. He can either sit down with SD and tell her the origins and ends of their marriage that is just factual (but age appropriate), or he can ask SD whether she thinks what BM is saying is true and help her logic it out herself, or he can reassure her that he isn't going anywhere, loves her, and these are adult issues that she needs to not worry about.

As far as this "routine" is concerned, if it's not court-ordered, end it. Take the cell phone during the day. Have SD call from DH's cell phone at night. SD is 10 and doesn't need her own cell phone because she's not going anywhere without her dad, so turn that shiz off and put it away. 

What you should have done is keep your mouth shut, it was certainly not your place, you realize you're just another one in the wrong, just like that BM was. 

Your DH sounds like a serial cheater/jumper- has he knocked you up yet?  Pretty soon we'll see a new stepmom complaining on here about you or you saying how your DH cheated and left you.  We see tons of those guys on here, several marriages/different baby mama's.  I don't see how you're defending this man.

Maybe that bm's mom knows nothing about her daughters circumstances, it certainly isn't yours to 'explain'- if your DH wants to share facts in a child approved manner, sure.  Everybody things they are different/special when participating in the same behaviors they dislike/criticize, I see hypocrisy on all sides here.  

Your dh can certainly limit/stop communications since he has her so little, btw, why DOES he only have her 2 days a month?  That sounds pretty crappy.  

We have a rule at my house  - "No Lies, even if the truth hurts". BM has told so many lies about me and DH, some we have had to defend in court. We never call BM nasty names, but we will tell SS the truth about who she is if a subject or issue (Crying after phone calls have happened at our house too) comes up. SS knows BM is a drug addict, prostitute, homeless (bed surfs with her many boyfriends), a liar, a manipulator, a felon, and the list goes on. When she does something horrible we explain in the the nicest way that she is a wicked bitch. She brought this up in court once, "They told (My precious baby) that I am a cocaine addict..." What actually happened (and why it's important to record all conversations with a crazy BM) is she mentioned, when she thought SS wasn't listening, that she still struggled with her addiction. SS asked me questions. I told him the truth. SS overheard her tell DH "Don't you think I had paying my bills on my back?" SS asked me what it meant. I told him. The judge agreed in both complaints that this was not alienation or talking bad about the other parent. The courts agreed that if it true and not said in a hurtful manner then we are not expected to lie to SS." There have been many other instances like these. 

So I probably feel differnt than most commentors. If BM thinks SS is old enough to hear a lie, we feel he is old enough to hear the truth.