Samsung E250 ring tones - format

Various reviews of the E250 say that it supports Mp3 ring tones.
Transferred a short unprotected Mp3 to the phone via bluetooth and the
Samsung PC suite. It plays well enough so it is getting there, but when
I ask it to set as ring tone it says "unsupported". Tried mmf and aac
but it dislikes that as well.

I am not trying to break copyright, children are fairly musical and
would like to compose their own tones. I am sure its me, not the phone:
any clues (the manual is spectacularly unhelpful on file formats). If
it can't be done I would like to know so I stop trying.

Thanks

--
Richard C