I think phones are stilled needed. They're still the most reliable form of communication. But I see most people switching to white lists. Anyone not in your address book will get sent to voicemail right away.
Imagine the future of mobile adware...where they get access to your contact list, generate generic names of similar ethnicity to your real contacts, and add fake contacts. Or they add additional numbers to contacts you already have.
"Oh, my friend Stan is calling. DANGIT NO I DON'T WANT A MEDICARE SUPPLEMENT PLAN!"