Leo says that some tablets would let him put a SIM card in, like the Samsung Galaxy Tab and the Google Nexus 7. That would essentially turn them into smartphones. He would need a GSM carrier, though, which is either AT&T or TMobile.
Another way to do it is to use Skype which uses voice over IP (VOIP). He can pay for what's called "Skype In" and "Skype Out", and he'll have a phone number that people can call him on like a phone.