The service, called BT Communicator with Yahoo Messenger, combines voice over IP (VoIP) technology with Yahoo's instant messenger service. BT says it will let users handle various communication methods, such as phone calls, Webcams, text messaging and IM, from their PCs through a single application.
BT also said that it expects to add the ability to make multi-way video calls next year.
Unlike enterprise-class VoIP services such as Cable & Wireless IP Voice, which use private networks, BT Communicator will send calls over the public Internet. This means that the telco won't be able to guarantee service level quality.
Despite this, calling a landline or mobile number using BT Communicator will cost just as much as if the customer had used their BT phone line. The precise amount will depend on which BT telephony package the user takes. BT charges 6p for calls that are less than one hour long, unless users are on a tariff giving free calls at some times.
Skype -- a peer-to-peer VoIP service -- is beta-testing a service called SkypeOut that will let users call landlines and mobiles worldwide. It currently charges 0.014 euros per minute (including VAT) for calls to landlines in the UK and to many countries including Australia, Argentina, China, France, Germany, Italy, New Zealand and Russia.
Calls to other VoIP users over BT Communicator will be free. BT Communicator will also work abroad, so if a mobile traveller installs the service on their laptop, they will be able to make calls to UK phones at domestic prices.
BT Communicator users won't get a seperate telephone number, so they will only be able to receive calls from mobiles and landlines on their PC if their landline is engaged. Communicator users will be able to call each other through the IM address book.
BT made its first VoIP move last December with the launch of an Internet telephony product for cable users. BT Communicator is the first product aimed at its own customers.