According to the Google (GOOG) Voice sign-up page, users now need just one phone number for the service. Previously, users were required to have both a number with a telephone carrier and a second one issued by Google. The search company says that it now offers "a single phone number that rings all your phones." The company adds, "We're excited to announce that you now can get Google Voice with a Google number OR with your existing mobile phone number."

This could be a nightmare for traditional phone companies. The upgrade will allow users to bypass the voicemail features provided by current telephone service providers. It also makes it simple for Google Voice customers to make cheap international calls using the Google infrastructure. The product is not widely available now, but Google says that it will be soon.

The Google Voice upgrade increases the pressure on traditional phone companies. The major telecoms make a great deal of money from the minutes that they sell their cellular customers and calls that customers make overseas. Google Voice could undermine both these sources of income.

Phones companies are now under threat from several quarters. Voice over Internet Protocol (VoIP), which is provided by most cable companies, has been an inexpensive alternative to landlines, and some VoIP applications work on wireless phones. Most cellular providers, if not all, are nervous that Google Voice will begin to do to them what VoIP is doing to traditional phone companies.

So far, Apple (AAPL) has not allowed Google Voice to be downloaded onto the iPhone, which is sold in the U.S. exclusively through AT&T (T). Of course, that's just a coincidence, not an attempt by Apple to stymie competition.

Douglas A. McIntyre is an editor at 24/7 Wall St.