The phones output is a stereo pair. With an unbalanced cable, your only recording half of it. It also sounds like you're recording a stereo track on the Tascam instead of a mono track.
You have two choices:
1. Use the cord you have and record as a mono track on the Tascam. Make sure you choose the correct side of the stereo pair to record, otherwise you'll get nothing. The result will be a mon track that you can pan around all you want. The down side is that if you are using the stereo effects on the keyboard, it might sound a little strance simce you only are recording one half of the stereo pair. Also, some keyboards that are designed for stereo hace the different octaves of the keys panned gradually aross the left right field (ie bass keys to the left, middle C center, higher keys to the right).
2. You can get a Tip Ring Sleeve (or balanced) cord and record a stereo track. This will yeild the fullest sound, but you won't be able to pan it around much.