What are average double opt-in rates for SMS? Is 38% too low?

Our lifecycle team at Quizlet recently implemented SMS as a new channel. We decided to implement double opt-in in order to safely collect written legal consent from our users. However, we are finding that about 56% of users who receive the initial double opt-in text confirmation don't reply back and are therefore not confirming their opt-in, so we can't officially subscribe them into SMS. Only 38% of users are actually replying back and confirming their double opt-in. The remaining people reply to opt out.

Since this is the first time we're implementing SMS, I'd love to get some insight from other brands who also use SMS. If you have double opt-in implemented, what does your double opt-in rate look like compared to the number of users who are initially submitting their phone number? If you're only doing a single opt-in, what does your opt-in rate look like? 

Any help is much appreciated, thank you!