Set Zable as your default voice line:
iPhone: Settings > Mobile Data > Default Voice Line > Zable
Android: Settings > Network & Internet > Calls > Preferred SIM > Zable
Check VoLTE settings:
iPhone: Settings > Mobile Data > Zable > Voice & Data > enable LTE or 5G
Android: Settings > Network & Internet > Zable > Preferred network type > 4G or 5G
Still having issues? Email zable-SIMcard-support@zable.co.uk and let us know whether all calls fail or just specific numbers, any error message you're seeing, and whether you can receive calls but not make them.