I agree with Dan "The Man" Lok and I'll go a little further. While you'll want to keep your onboarding as simple as possible and eliminate any barrier to capturing that email... don't YOU make the decision for them. Rather, let it be OPTIONAL. Let them decide! However, I do suggest inspiring their trust and giving them some incentive to share it. Ditto to what Dan said, i.e. let them know you won't be spamming them or sharing their number and that primarily, it will allow them easy access to resetting password, etc. BUT also firstly, incentivize them to share their number by letting them know that occasionally you'll run special VIP first-come, first-served promotions via SMS and you'd hate for them to miss out! I'd also like to suggest that you don't get all crazy with psychographic insights, etc. It's easy enough to read up on good UX versus bad UX (check out useronboard.com for some cool teardowns and more). As a strategist, I'm very keen on insights, but we all know why folks don't want to share their phone numbers -- think about why you might not want to share yours and address that "psychological" barrier in the most succinct way possible. We also know that mobile marketing includes SMS/Text Message Marketing and there are plenty of fabulous stats available proving why you'll want to capture mobile numbers if you can. Let us know how it goes!
P.S. run some A/B Testing - tinker with your copy and design and see how that works too!