Hello!
My name is Elena.
I'm a representative of Unlim company. Unlim company specializes in serious projects, especially for commerce field. We have a qualified team of programmers and thoroughly implement each application. We've made 4 mobile applications as a team (native Android and iOS): for water-delivery company, for courier service (tracking and coordination), for restaurant food delivery company (to order dishes) and for medical clinic (appointments, push notifications, prescriptions).
We are very interested in your project! I can estimate your project as 3-4 days of work for our iOS specialist. We have one remark: iOS system doesn't allow to take the phone number from the mobile device settings. The user should input it manually.
The answer to your question: the best decision is to generate a hash-string (md5 or sha1-2, based on unixtime and phone number) for each registration, save it on the server and compare hash-strings on server with hash-strings in apps upon app request.
I'm always ready to consult you to solve different tasks as your personal manager.
Best regards,
Tyuneva Elena