I went to New Caledonia recently and brought along my 2degrees SIM as their website says they support voice/text roaming in New Caledonia (whereas neither Spark/Vodafone offer roaming there for prepay). I found that roaming wouldn't work at all. The 2degrees SIM would only report 'Emergency Calls Only' and wouldn't connect to the network there at all. Luckily I still had VoWiFi so could still send/receive SMS that way for 2FA etc.
As I used the same phone/SIM to successfully roam in Australia only a few months ago I don't think it was a roaming settings issue (as I would not have been able to roam in Australia otherwise). I checked and made sure roaming etc was all enabled in the settings.
It wasn't a band compatibility issue as I got a local SIM which worked just fine on the Bands 3 and 20 used over there. The whole week I was there I had 2degrees in SIM 1 reporting 'Emergency Calls Only' the whole time and the local OPT/Mobilis in SIM 2 working fine.
Will be emailing 2degrees but posting here to see if any others have experience to share?