The server refuses to perform the request using the current protocol but might be willing to do so after the client upgrades to a different protocol.
In moving towards a more secure posture with regards to TLS, we require that all new accounts must use TLS v1.2 or later. All customers must use TLS > 1.2 starting June 26, 2019.
Customers running older operating systems or legacy network software may need to upgrade their systems to be compatible with these changes. If you've received Error 20426, you likely need to upgrade your system.
To learn more about these changes, see our support site's FAQ on Twilio REST API's TLS and Cipher Suite Security Changes
To learn more about Twilio's TLS and cipher requirements, please reference this support article where you can learn more about understanding failure signatures, which components to upgrade, and verifying your upgrade.