I'm with you! I've had no luck so far making my own ringtones for my 7105t.
I've done some digging and found that the Oki ADPCM format is just the ITU-T G.721 standard, which has been superceeded by the inclusive ITU-T G.726 standard.
http://www.itu.int/rec/recommendatio...nt=T-REC-G.726
They have also put some C code together for a sample codec:
http://www.itu.int/rec/recommendatio...nt=T-REC-G.191
However, both of those are not free or cheap. The software package is like $50 USD. The SW manual is free though.. worth looking at.
Another site I found has a G.726 codec in C for free here:
http://www.dacsindia.com/products/g726.htm
http://www.dacsindia.com/downloads/downloads.htm
You have to sign up and they send you the download link, but I haven'y recieved anything yet.
This format dates back to 1988! So I'm really suprised that there is not more info and utilities out there for it.
Keep me in the loop if you find some codecs. I'm interested in coding something up as well ;)
EDIT: Here's an interesting little demo showcasing the ability to convert from WAV to G.726 (ADPCM) and then from G.726 back to WAV... you can even use your own files... too bad it doesn't have a G.726 output.