I have a page in which users are an employee of a charity. They can / should enter information about their organization including phone numbers. It occurs to me that some organizations may have dozens or even hundreds of phone numbers. I do NOT want to have to place that many <input> tags on the page since most users will never need them. I thought it would be nice if there were some sort of input device that can be expanded as needed like a spreadsheet.
If there is no such control, I want one. I want it very similar to an <input type=text> control because I want to limit the length of the user's input and even the content, if possible. Also, the cells need to be named as they are created so the server can know they exist and can extract their value.

Two things spring to mind, jquery and a server side scripting language. The latter is optional given your question since jquery would adds more html input fields on the fly.

But I'm guessing you would eventually need to store those values in the db.