It's in the £5.99 iWork's operators manual for numbers for IOS
"When you type or paste a web address or email address in a text box or shape (but not a table), Numbers recognizes it and autoformats it as an underlined hyperlink. (The link must be followed by a space or punctuation mark for Numbers to recognize it.) To open the link or copy its URL to the clipboard (for later pasting), tap the link to make a pop up window appear, and then tap Open (for web addresses), New Message (for email addresses), or Copy. Tapping Open switches to Safari or Mail. To edit or remove the link, tap Link Settings."
I've removed that comment now but it's quite clear on what it can and can't do. Does the max iOS version work on the iPad?