Given that the power supply is rated for both less voltage and less current than the amplifier is listed as requiring that may be one problem.
In addition, since it seems that the input connection is directly to the microphone circuit of the phone, and because none of the phone circuit is either ground referenced or shielded, that is probably another source of hum. My suggestion to use an audio transformer and connect the primary across the ear-speaker was not just some idle rambling. It is a way to avoid picking up hum from phone circuits. I have used it and I know that it works, at least with the correct transformer, one that has at least a medium impedance of both primary and secondary.