For a little $1 iPhone app, Line2 sure has the potential to shake up an entire industry.

It can save you money. It can make calls where AT&T’s signal is weak, like indoors. It can turn an iPod Touch into a full-blown cellphone.

And it can ruin the sleep of cellphone executives everywhere.

Line2 gives your iPhone a second phone number — a second phone line, complete with its own contacts list, voice mail, and so on. The company behind it, Toktumi (get it?), imagines that you’ll distribute the Line2 number to business contacts, and your regular iPhone number to friends and family. Your second line can be an 800 number, if you wish, or you can transfer an existing number.

To that end, Toktumi offers, on its Web site, a raft of Google Voice-ish features that are intended to help a small businesses look bigger: call screening, Do Not Disturb hours and voice mail messages sent to you as e-mail. You can create an “automated attendant” —“Press 1 for sales,” “Press 2 for accounting,” and so on — that routes incoming calls to other phone numbers. Or, if you’re pretending to be a bigger business than you are, route them all to yourself.

The Line2 app is a carbon copy, a visual clone, of the iPhone’s own phone software. The dialing pad, your iPhone Contacts list, your recent calls list and visual voice mail all look just like the iPhone’s.