You let someone else register your domain name
Your domain is more important than your phone number
Your domain name is extremely valuable and irreplaceable. You can lose it forever if you're not careful, along with all that money you've put into marketing and printing. And you won't get a friendly "The Domain Name has changed" message like you do when your phone number changes.
YOU need to register your domain name, not your web developer or your consultant. You can have them walk you through the process, but you need to do it yourself, because if you don't, then you may well be developing a brand around a domain that your consultant owns, not you.
Your developer has a big stick if you have a dispute and they control your domain name. They control your email, your web presence, and your ability to be in business, all because of a $10 a year domain name.
Keep your old email address
You need to make sure that you keep the email address you used to set up the domain, so that as it comes up for expiration, you'll get notified. There's nothing worse than finding your site, and your business, are down because someone forgot to tell you to pay the registrar.
Of course they can't send you information for how to renew your domain if the only email address they have for you is email@example.com.
Buy it for a long time
Getting a domain name for 10 years is cheap in the grand scheme of things. Heck, you can get 100 year registrations. If you buy it now, and tie it down for virtually forever, you'll never run the risk of it expiring when you aren't watching.