Wash,

This would largely depend on what kind of PBX you are using. If it's the conventional Panasonic then they mostly have an audio input that can be connected to a CD/MP3 player and programmed for playback.

If it's another manufacturer - then it would be necessary to check whether similar functionality is available.

If it's an IP PBX then a config for music playback from MP3 should be possible (once the relevant files have been uploaded).

If it's none of the above and you must have a external device for this - I would suggest you talk to Technology Today (used to be based in Westlands) or Telebell(? who I think should still be in CBD).