If an SP has her number displayed on AW (or whatever site) I would think it's fine to text her, unless her own profile text explicitly says otherwise.
Ignore the AW boilerplate about "SMS messages will be ignored".
Keep it relevant and to the point. My basic template is "Hi, my name is X, responding to your profile on AW. I'd like to see you for a 1 hour incall at 7pm this Friday. Please let me know if that works for you".
These ladies are looking for business, it's unlikely they'll object to genuine enquiries. Just do yourself a favour and don't send any text message that would make her reasonably assume you're typing with your dick in your hand (and either no intention of booking, or if you do you're a boundary pusher). Long back-and-forth exchanges, requests for pictures, "tell me what you're going to do to me", stuff like that.