The T10(H) and T20 telephones do have a FLASH button for call waiting features.
If you have call waiting service and you hear the call waiting tone, press FLASH to accept the waiting call. There may be a short pause, and then you will hear the new caller.
To return to the original caller, press "TALK/FLASH" again.
Please note that call waiting is a service offered by your telephone provider, and you will need to contact them directly to subscribe.