iPhone/iOS - Change Mac Addres

Supported version: All iOS version (3,4,5, 6, 7)

Change mac address:

nvram wifiaddr=XX:XX:XX:XX:XX

Restore to the original mac address:

nvram -d wifiaddr

Restart iPhone after executing change/restore command above to finish the process:

reboot

iPhone not displaying incoming calls on call waiting or giving an option to answer

Solution:
Go to Settings > General >  Reset > Reset Network Settings