You can hide your caller id (phone number) for one call only!

Tested and works in these countries:
USA (At & T and Verizon also), Canada, Mexico,Argentina,Brazil
Germany, Italy, Belgium, Spain, Russia,Romania, Hungary, Czech rep.
Arab Emirates, Pakistan,India (seems like not working for prepaid cards), Malaysia, Hong Kong,Singapore (not singtel)

Australia
And everywhere else.check if the service is available for you with yot provider!
- How to use:
Pick a contact or enter a phone number.
Set your prefix code based on your location(country). The code will be generated. If it contains # or * only a dummy number will be called. After the dummy call ends, you can paste the code in the dialer!
If the generated number does not contain special characters, it will be called immediately. (like in UK). See prefix codes on home page. Default settings are for US AT&T.
Other codes (if you cant find your country, try generic):
USA AT &T *67
USA Verizon *67
Generic GSM #31#
UK 141
Germany *31#
Hungary #31#
Australia 1831
New Zeeland,Telecom 0197
Israel *31
Hong Kong 131
Denmark *31*

Video tutorial on youtube.
Hiding will not work if the one you are calling has TrapCall or likes.