You seem to have enabled an option (perhaps within a calling plan from your landline service provider) to always withhold your number when making outgoing calls. Try calling your mobile again but putting 1470 in front of your mobile number. With BT (and possibly with other providers as well) that overrides the number-blocking function.
You need to go into the "call settings" in your mobile.
Then you need to select "caller ID".
There will be 3 options:
Network default; hide number; show number.
You've prob got it set on "hide number" - you need to change it to "show number" or "network default" (assuming that the network default is to show the number).
The above may vary slightly, depending on make and model of phone ;)