How to make outbound calls on iPhone from 1-800 number we have through Twilio

I would say simply: a patience. Build the momentum much before the release, months or years. Release when people rave about your app.

NASA is famous for writing the most bug-free code in the history of programming. They do it by being fanatics about test-driven development. They write tests for tests. If you want to have a rock solid app, you have to put everything else aside and focus only on quality. Every time a new build goes out, every single member of the team has to test every tiny nook and cranny and write tests to mimic how they're testing it in real life. The goal is to start small with a solid core and get it functioning error-free before building other features. If you hold yourselves accountable to make feature incredible before you begin building others then you'll never have to go back and clean up something later. In short, try to do it "write" the first time :)

Optimizely is a popular testing service that lets you do this. See: https://www.optimizely.com/mobile/ and http://developers.optimizely.com/ios/

Re-visit your marketing strategy. Which is to say - re-visit your business model (which should very directly point you in the direction of your highest probability marketing strategy). And (get ready to not like me for saying this...) stop focusing on those numbers. The number of visitors per month and the number of downloads are only relevant as they directly relate to conversions and sales. And those things are only relevant as they directly relate to bottom line revenue and margins. It's easy to get caught up in "vanity numbers"... those numbers that "feel good" and provide you with a sense of validation. But until and unless they translate into bottom line income for the business they are just numbers that make you feel good (versus wisely chosen key metrics that indicate the health and viability of your business). If this was helpful to you please let me know how else I can be of assistance.

Ultimately, it depends on the value the app has for the buyer (not how much money you put into developing it). Unfortunately there are many reasons for a buyer to purchase an app and so there are many ways to value an app (a multiple of yearly revenues,number of active users, etc.) . A good solution to this problem is to post your app on an auction site such as http://www.apptopia.com/ and let the marketplace give you an answer. Good luck with your sale.