Sure, you can rent access from Twilio, in the same way that you can rent access from Facebook. It’s extremely unlikely that you could ever become a full participant in either network.
Okay, so there's a high barrier to entry. It's pretty hard to become an ISP too. That doesn't make either a walled garden.
It's not so much about whether I personally could federate with the network, as it is about whether some reasonably large number of other entities could. I don't have exact numbers, but it seems there are 1000+ phone providers worldwide.
Sure, you can rent access from Twilio, in the same way that you can rent access from Facebook. It’s extremely unlikely that you could ever become a full participant in either network.