It seems like they're being much more cautious nowadays than in the past. And while no guarantee, you can most likely help your cause by sending a detailed text with lots of context. Something along the lines of "Hi, I saw the ad for Aiko on Adult Search, and I would like to be screened to make an appointment. My name is XXX, and I have references that I can provide, as well as handles on review sites ABC, XYZ, etc. Let me know if you require any additional information, so I can be screened to make an appointment". Something similar to that.
Obviously only include info above that you're okay with sharing, but I would say that being as detailed as one is comfortable with, would go a long way for them to respond. In other words, try to make them comfortable with you for them to accept you as a new client.
I don't know the details of their current screening, and I'm not speaking for them, but it's also possible (and some places and provider do this) that when a prospective client texts them, they look up the cell number on a reverse phone number site, and if they see that it's not a major carrier and it's just a VoIP throwaway type of phone number, they might disregard it and not reply. Again, merely speculation on my part.
It's also possible that they're only open to existing clients at the moment, and in that case, you would just have to try at a future time...these things almost always open up again. Or, if you know anyone who is a current client and is willing to vouch for you, that'd likely be the only other way.