Sure, I know a few people that use iPod touches as their “main phone” using VoIP apps. But they’re of the paranoid hacker type and want a device that’s easy to lock-down from phoning home 24/7. It works out for them since they don’t tend to make or receive actual phone calls much.

Only issue is most of these VoIP services can’t be used in a multi-factor authentication scheme since they don’t support short code SMS.