I agree with gr8Dad. This is absolutely a form of abuse. And we have been in these shoes, so I can really relate to the poster.
enyawb31, get the texts off that phone as evidence, and haul your ex to court. What she is doing is very unacceptable. You can legally ban her cell phone, and limit her exposure to the kids during the visitations. Also, you may want to time limit their calls to her from the sounds of it.
Back before we had the cell phone issue, we had to actually remove the cordless phones from our house, and change to a corded phone in our kitchen only. Otherwise, the biomom would tell the kids to take the cordless phone to their rooms, and then try to work them up into causing trouble.
Also, I would recommend buying cell phone for Dylan while he is at your house. One you are able to manage. You can set it up so there are time limits, and other forms of controls. And if he does text her, you can use those texts as a way to hold her accountable. Last, it will keep your son from becoming super rebellious. Kids at his age become a bit crazy when it comes to cell phones.