Modding
Since K750i and W800i are essentially two phones with exactly same hardware specifications, soft modifications are available to convert a K750i to a W800i by using flashing software and the firmware files of the W800i model. Such modified phone will be able to use the Walkman feature which is available to W series phones only.
Such phone can also use the flight mode option which allows the phone to work only as a media player while other functions, like cellphone radio system, are turned off. This option is essentially usable when in places where mobile phone transmitters must be turned off. The Activity Menu will not be present which appears when the button between the left and right softkey is pressed. Instead the Walkman player appears when it is pressed. This modification will revoke any remaining warranty for the phone.
Modding also called as flashing is done by software such as XS++ or Far Manager.
Read more about this topic: Sony Ericsson K750