Is Plivo free to receive inbound SMS from local numbers?

I am thinking about creating a project that will hopefully be receiving millions of inbound SMS messages per day if it became successful.

Plivo claims to offer free inbound SMS at least from local numbers.

Are there any caveats to that?

I am wary because "unlimited free" plans typically have hidden clauses that don't mean what they advertise.

Just curious if anyone has run into a situation like that, or if there is another better option to plivo?