How can i keep my number and where to call
To bring your mobile number to giffgaff, you need to:
Remember to backup your contacts and data on your old SIM before proceeding with the transfer..
This guide provides full details of the process http://community.giffgaff.com/t5/Getting-started/H
Please note: You can only transfer (port) a number from one UK network to a different UK network. You cannot port a number from one network to the same network (e.g. giffgaff to giffgaff), or to/from a non-UK provider's network.
If you are porting in your old number from a different network you need to get a PAC code from your previous supplier. Then go here:
This link should tell you all about it.