[barnabask]

Same here, but with US postal codes. Having grown up on the west coast I didn't realize some started with a zero until embarrassingly late in life.

[roywiggins]

The best rule of thumb I've seen for this is that unless incrementing a value makes sense, probably use a text field unless you have some other really good reason. So: auto-incremented IDs yes, arbitrary IDs that you don't control no, phone numbers no, SSNs/PINs/ZIP codes, also no.