I’ve been looking for such a device for some time now. I was considering trying to make one, then the ZSE50 was released.
I post this to say the custom sounds are extremely easy. Simply create an MP3 file with the sound you want to use as a custom track, then connect the ZSE50 to a PC with the supplied cable. The ZSE50 then acts like a USB drive, you simply upload your file and your good to go.
I also found for custom messages it’s better for me to use one of the text to speech sites so I don’t have to use my “not good for answering machine” voice.