Having the same issue as the other users here:
After recontracting to the Sim Only $25 plan this weekend
VoLTE has stopped working, (was working fine previously on my Starhub CIS plan)
Thanks for contacting us. As this requires us to refer to your account details, please direct your query over to Facebook messenger (http://m.me/starhub) or Twitter Direct Message (https://twitter.com/StarHubCares) along with the registered name, mobile number, Hub ID/email address or service address so that we can log a case for you and respond to you there.
Only after I contacted their twitter account and they went through their list of inane question as they did not read my messages at all.
And then finally they got their backend to reset my volte provisions and it was fixed instanty once they _finally_ did that
Seems to be the same issue for all users who recontracted to the sim only promotion over the weekend
Good to know that it finally works for you!
Amazingly, mine are still down. I guess I will have to take up the other social media routes as well.