Personally, I'd like to avoid vendor lock-in as much as reasonably possible (Yes, I know we have Stripe and Client Portal Microsoft SSO).
The amount of time spent developing an integration just for Twilio would (in my opinion) be better spent developing additional API endpoints that anyone can use for anything they like.
My ideal picture of this is that you have your own Powershell/whatever script that queries the ITFlow API for things like new unassigned tickets/expiring licenses, etc and then takes custom actions (e.g. delivers additional notifications via Twilio/whatever). I get it's not as "out of the box", but it'll be far more flexible than forcing people to a specific vendor.