Comment by quit32

Odd use of Twilio for just a SIP Trunk provider. They also offer programmable voice SIP, a product that is a distinct offering from the Elastic SIP trunking used here. With PV SIP you can directly register the Grandstream HT801 ATA to a programmable SIP endpoint on Twilio and move all the programming logic, including text to speech, to a Twilio serverless "function" and completely cut out the need for a RasPi and Asterisk.

Also FYI for those who do want to use Asterisk behind CGNAT and cannot open ports - if you use PV SIP you can also register it as a Trunk in Asterisk and it can then receive inbound calls over the connection established by the outbound registration with simple a simple Twiml bin that routes whatever Twilio numbers you want to the SIP user you used to register the Asterisk's SIP Trunk. Asterisks can can then use inbound number plan to further route the inbound call as needed.