if i put an Orange sim in (this is the network that the phone was locked to before it was unlocked by the guy that had the phone before me), the phone will always play the "Desk_phone_lo" ringtone when it rings if i select a .WAV .AMR .AWB file as my ringtone, if i select another "built-in" ringtone, it will play that...

if i put any sim in other than an Orange sim, the phone will play any ringtone i select when it rings (even the .WAV .AMR .AWB files)

sounds to me like it's the Orange firmware which is to blame (yet again) for all these issues/problems... (i've had other problems with it before) the phone might need to be debranded, or better still, to save messin about... replace it...