Until a jailbreak for the iphone 4 is out, users can add a speed dial icon to their home screen for convenience.
Press the Safari button on your iPhone. This will open up your web browser.
Press the URL bar once your web browser opens and type in the area code and telephone phone number and .tel.QLNK.net. If you were to use the Giffgaff Voicemailnumber (443) it would look look this: 443.tet.QLNK.net Press enter now.
Once it's dialling press cancel when you are given the option to either cancel or call the number you entered.
Press the + (plus) sign at the bottom of the screen when you receive the "add this page as a WebClip to add a speed dial entry to your iPhone Home Screen" pop-up window.
Press the "Add to Home Screen" button.
Name the telephone number (for example Apple) and press add. An icon of a telephone with the name you chose underneath it will appear on the home screen of your iPhone.
Press the icon to make the call.
I hope this helps guys!