Why does the keypad show strange entries on my iPhone?
If you paste text, it translates the letters into numbers as represented on the keypad.  For example: 1800 BUY NOW becomes 1800289669

If you've copied text instead of just a number, or if you've copied a link behind a number, you might actually be pasting a web address like http://www.apple.com which is changed to 488799927753266 after stripping punctuation.

Go to the Notes app and paste there.  What is actually stored in the clipboard?