The Skype caller identification (caller ID) option lets you set up an existing mobile number as a presentation number that people see when you call them. This is a great step forward in integrating Skype into your existing comms set-up. An issue that is a pain when using an additional telephony service for outbound calls is that it either doesn't present a caller ID at all or it presents some strange number that will either confuse people or prompt them to try to call you on a number you don't want to receive calls on.
I've set my Skype caller ID to display my Truphone number which is my preferred number to receive calls on.
