[ta1234567890]

Verizon also blocks messages based on the urls they contain. Not sure about specific TLDs, but surely whole domains. Discovered this by running a service that sends a lot of messages through Twilio. Not sure if you would ever be notified of the block when sending from your phone.

In my opinion is not really to block spam, but instead to push message senders to buy the carrier's more expensive shortcode option.

[rsync]

... which can’t be reached from twilio, since twilio Numbers are not actual mobile numbers.

[zachrose]

What is it that can't be reached from Twilio? Carrier short codes?

[rsync]

Correct. Twilio does not give out any proper mobile phone numbers. Therefore you cannot send SMS from a twilio number to a short-code.