Look in the V6 call control API guide and turn to the section entitled 'call transfer'. This section describes the call control procedure required to perform a call transfer if you are configured as the 'user' side. The section also specifies the supplementary services that have to be supported by your exchange in order to perform a successful call transfer.
Alternatively, if you are configured as the 'network' side and want to perform a call transfer, turn to the Appendix entitled 'network side call transfer'. This appendix can also be found within the V6 call control API guide.
If you are using a CAS protocol the only method of performing a transfer is by using the 'tromboning' or 'hookflash' methods.