In other words, Jajah emulates your phone number to make a connection via the web, without accessing your mobile network.
Usefully, Jajah lets you try the service once before you sign up, but only by calling from a landline number to another landline.
Once you've registered with Jajah, you choose the phone number you want to make calls from (either a UK landline or mobile number), tap in the phone number you want to connect to and activate the call. Your phone will ring to confirm you've entered the right number. When you answer it, your friend's phone will ring.
The VoIP service has a dedicated mobile website, which lets you log in directly from your mobile handset and then call any telephone number in your Jajah address book for the same price.
Jajah also offers low-cost calls to international numbers. Once you've logged into your Jajah account, enter your friend's international number and press ‘Get Local Number'. Jajah gives you a local number (relating to the country you're calling from) that will connect you to your contact.
You can add a calling button to your blogs and social-networking pages. Although this won't display your phone number or contact details, it will allow people to call you. You can even block or reject certain numbers or identify times when you will accept calls.
It's free to call other Jajah users on a UK landline or 2.2p per min to non-members. Calling a mobile phone from a landline costs 13.3p per min. To make calls from a mobile you must first register your mobile number. Calls will then cost 13.3p to a UK landline and 24.5p to other mobile phones. International call costs vary, but expect to pay around 2p from a landline and 13.2p from a mobile.