Possible to playback RFID card using NFC functionality on phone?

Solution 1:

Assuming the RFID system you are talking about only expects a key, and then does a look up in the user db.

Usually, a new UID is generated for each NFC transaction by phones. Some users have been able to maintain a static UID sent from the phone to the RFID receiver. A static UID will then act to send a constant "key" across to the RFID receiver instead of a generated one, everytime you try to swipe the device.

To obtain the static UID, you need to change the firmware on your phone, to only send one UID. Users on this forum have achieved it on the Google Nexus S and EVO 4g LTE.

Happy days!

Solution 2:

You can melt the card with acetone, which will get you the chip and antenna. You could then stick them inside your phone cover and use it as an RFID card.