For reasons, I wanted the "running out of credit" music beeps that the Taipei metro gates give when swiping the travel card. I ended up recording it then turning it into a message tone (using Garageband on an iPhone). It sounded ok, but luckily I eventually found someone else had done a better job and grabbed an mp3 online.

I did a quick search (Youtube) and found lots of call bell sound but none seemed to be the BA one unfortunately. Record it next time.