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? Contact us here using email address you signed up for Zable Mobile, 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.