BeepBeep Yes and No, if disappeared suddenly, yes you will loose your number but that's the issue with every VoIP and non VoIP service provider.
But considering their history if they are about to shutdown they will probably give the users a notice so everyone can port out their numbers to some other service.
If they get hacked probably they will have some kind of backups or DR plans to restore their operations, but during the recovery, services may not be available, this thing is common for everyone online services. I don't have any insider knowledge to confirm on this.