In an unusual deal, BT, the largest carrier in the United Kingdom, and RingCentral, a hosted voice service, on Tuesday introduced a web-based phone system for small businesses in the U.K.

The deal is unusual because large carriers have traditionally steered clear of web-based telephony firms such as Skype and Vonage in part because they see them as interlopers and direct competitors for subscriber revenue.

But RingCentral, which focuses on small and home offices that want phone features such as voice mail, fax, and multiple lines that make them appear to callers as larger, professionally-run firms, believes it is different to Skype.

"Skype offers a solution that is competitive with the carriers for long-distance service which is where the carriers make their money," said Vlad Shmunis, RingCentral's CEO. "We offer a service that is complementary to the carriers."

Serving very small businesses has always been a problem for carriers because business phone systems are expensive and usually out of the financial reach of most home-based businesses. Most home-based businesses make do with multiple individual home phones, according to Moe Tanabian, a principal with IBB Consulting.

"Phone systems for small businesses are actually more expensive than for large firms because of the economies of scale and RingCentral and companies like it provide a less expensive way for carriers to reach this market," Mr. Tanabian said.

RingCentral hosts public branch exchange (PBX) services for small businesses entirely on the web. The PBX, a business phone device, is virtual so there is no need to install any equipment at the customer site.

U.K. customers will pay £15 per month for the service called BT RingCentral which will feature multiple extensions, voicemail, automatic call screening and forwarding, click-to-call, call logs, and message alerts.

But Redwood City, California-based RingCentral faces tough competition from hosted services such as New York City based M5 Networks, and VoIP services such as Vonage and Skype, both of which woo home-based businesses.

But carrier partnerships provide a significant advantage because of name recognition, reputation, and customer access.

The growing penetration of broadband services around the globe will make this a very competitive market, Mr. Tanabian said, so he expects to see many more deals involving hosted voice firms and carriers.