Teferi's Protection says:
Until your next turn, your life total can’t change and you gain protection from everything. All permanents you control phase out.
I am not phased out, and the cards in my hand aren't phased out, so can I still cast spells before my next turn? I realize that my lands are also phased out, so I can't tap them for mana. But if I tap all my lands before casting Teferi's, can I spend surplus mana until it disappears at the end of the current phase? And can I cast spells with zero cost?