Does anyone have any hints as to how to get Beeper Mini working with a UK SIM?

Whichever SIM I try, when it gets to the phone registration stage (sending an SMS to Apple) I get a message “ Something went wrong -please check your network settings or try again later”

Things I’ve done:

Checked network settings Tried again later Tried different SIMs with different numbers Cleared Cache and Storage. Uninstalled and reinstalled app Turned phone off and on. Tried a different phone.

Is it just buggy as hell rubbish?