1. Sign-up costs

There are several services that will allow you to sign up free, including Skype, SIPGate and VoIPCheap. Other providers such as Tesco and Vonage have signing up fees.

2. Accessories

You will not have to spend a penny to use some VoIP services. If you are signed up to Google Talk, you will be able to make calls using your PC, provided that an internal microphone is installed.

A headset and microphone can be purchased from computer stores for about £15. But research by Which?, the consumer group, found that the best phones cost £40 to £100. If you sign up to Vonage, you will need to fit an adaptor to a USB port in your PC, which is available free. Users can speak via their phone, which is connected to the adaptor.

3. Types of service

Free VoIP services offered by companies such as Google and MSN will allow free calls only to other members and not to mobile phones or landlines.

If you have signed up with BT, you can make free PC-to-PC calls but you will be charged for calling mobiles and landlines.

4. Call charges

The price comparison website uSwitch says that daytime national call rates, excluding those made between users, range from 1p to 3p a minute. Some VoIP services also levy a monthly fee, typically between £2.99 and £9.99. Customers usually obtain free calls on these deals.

5. Wi-fi

If you use broadband via a wi-fi connection, you can purchase a hands-free wi-fi phone. Vonage offers a portable phone for £89.99, which can also be used at “hotspots” outside the home, such as those in cafés and at airports.

6. In an emergency

Most VoIP packages will not allow you to call 999 — BT and Vonage are notable exceptions. Ofcom is currently updating its guidance to the industry to encourage companies to allow users to call the emergency services.

7. Think of a number

You choose or are assigned a telephone number. Because you are bypassing the landline system, your number is not restricted to normal area codes.

8. Where can I use VoIP?

Like e-mail and mobile services, you can use VoIP anywhere broadband is available.

9. Online only

If your PC is switched off, you may not be able to receive or make calls on some services. The BT service, however, will accept calls even when a PC is switched off.

10. Sound quality

Which? found that the sound quality varies between VoIP packages. Skype was found to be blighted by delays while BT had fewer glitches. Delays can also occur during peak times or over a slow internet connection.