For all practical purposes, you can't: there are lot less hash codes than there are strings, so there is more than one original value that would give your same hash code.
Hashing is, in reality, a one-way operation. If someone refers to a reversible hash, this isn't a true hash (because a hash by definition reduces an input set into one of a smaller number of output values). The closest operation to what you describe might be an encryption function - this would allow you to reverse the operation - but this is unlikely to generate as small a number as the 10-digit output in your question.