How do you block a cell number of iphone 4 - Apple iPhone 4S

You can do it directly in the Phone app on your iPhone. Check out this post to see how to block a number on iPhone: http://www.leawo.org/tutorial/block-numbers-and-find-blocked-numbers-on-iphone-1334.html

You can block a number on iPhone via messages app. This works really well for blocking phone number on iPhone, like phone numbers from some unknown spammers, advertising companies and internet marketers bothering you with promotions and offers for products that are not at all related to your interest. These steps will block only those unknown callers that you choose to block. The detailed steps are as follows:
1. Go to the Messages app.
2. Tap on the target message from the spammer.
3. Choose "Details" on the top right hand corner and tap on it.
4. There will be phone icon and a letter "i" icon across from the number. Tap on the "i" icon.
5. Scroll down to the bottom of the page and then tap on "Block Contact".

Note: Once you press on Block Contact, the phone number that you just blocked won't be able to make phone calls, send text messages or Facetime you.

http://www.leawo.org/tutorial/how-to-block-a-number-on-iphone.html

You can block any number on your iPhone by just installing an app called "iBlacklist"
There you have many options, and with them there's an option of blocking a cellphone number.
You go to Blacklists - General BL. There you have '+' in upper right corner. From there you can add the number you want from contacts, recent calls and messages or write it yourself.