I'm experiencing the same issue with Gluu 4.2.3 on Ubuntu 18.
I've submitted a support ticket to Twilio in the hope they can provide more detailed information about what is wrong.
https://www.twilio.com/console/support/tickets/6402223
I can send to US cell phone numbers, but not UK or AU cell phone numbers.
In Twilio the geoblock us set OK:
https://www.twilio.com/console/support/tickets/6402223
I'm using a twilio trial account and I've verified all the phone numbers I'm trying to use.
I'm using twilio-7.17.0.jar which is what comes with gluu 4.2.3
any suggestions appreciated.