It's a CELP codec which is used by Philips recorder devices. I've been searching for hours trying to find a codec let alone a converter.
There is a free audio converter called Switch Sound which can work well most of the time but for some reason it doesn't work well here.
Tried
https://www.dictation.philips.com/home/?redir=none to find software that will at least play the file, but that didn't work either.
My thought was to find a physical Philips player and place the files in there and play them back and record that. Like a voicemail machine. Thoughts anyone?