The easiest way to perform transfers with the siemens gigaset range involves changing a setting in the web interface first. 

Browse to the IP address, login and click on settings".  Go to the "telephony" section and click "advanced settings".  Look for a setting called "transfer call by on-hook" and set t to "yes", then click "set".  You can close the web browser now.

Now to transfer a call, there are two types of transfer:

blind transfer:

- with a call in progress, press the "ext call" button, dial a number
and press send.

- when you hear the ringing tone, hang up.

- The original call will have been transferred to the number dialled.

attended transfer:

- do the same as above but when you hear the ringing tone, wait for the
other person to answer.

- announce the call and then when ready, hang up.

- the first call will now be transferred to the second call.

Note: for any transfer to work the service provider you are using needs
to support SIP transfers.