Ask HN: What are the best services for sending texts to real numbers?

Hoping to get some advice from other HN startup people.

I'd like to send SMS messages to users that are 100% opt-in and welcomed by the users. Nothing to do with spam.

But I only want to send SMS messages to users with "real" cell phone numbers because I'd like to also use it as a abuse-reduction mechanism.

And I'd like to restrict costs so that I'm not spending more than maybe 1-2 cents per SMS, even if that requires limiting which users can receive them.

I'd like to be able to send 5-10k (now) to 100k (later) SMS per day.

I've checked out some services but it seems pretty difficult to compare them without actually using them.

Hoping some people here have some good recommendations.

Thank you!