Wi-Fi Calling disappeared?

Took out a new plan with Lebara last month,
A few days after starting use of the SIM I received a message stating that Wi-Fi Calling and VoLTE were enabled.
For the first week of that text, Wi-Fi calling worked fine, however, now, it's seemed to have disappeared.
I've attempted to re-insert the Lebara APN settings and re-insert the SIM but nothing's changed.
Has anyone else had a similar issue, and how did you fix it?