Disclaimer or Terms and Conditions
All sites that obtain personal information should use some kind of disclaimer or terms of conditions on submitting such data to the system, this varies from site to site and you should do a little research on what things you should be mentioning. If its for a big project then I suggest a legal adviser. It's important to note that even though you have a disclaimer or terms and conditions you can still be liable for damages which I'll cover in the next paragraph.
Data Protection Act
Disclaimers or Terms and Conditions do not make your company, or yourself immune from certain laws or acts, you can still find yourselves in trouble if you are not taking reasonable steps to protects peoples data. Data Protection Acts vary from Country to Country while most are the same and mention that reasonable steps should be taken to secure peoples data, some say you shouldn't keep UNUSED data on a computer for longer than X Months or Years.
If you are storing UK numbers then its best to check http://www.ico.gov.uk/for_organisations/data_protection.aspx etc, and so on. Personally its best to download these and store them offline or make sure your internet site is super secure.
How To Store Data Safely
It's impossible to say without a list of your server, the software, and everything else. I recommend you Google how to secure a website and ask questions on Security at https://security.stackexchange.com/