Help needed with MP3s on Mobile phones

I have downloaded some MP3s onto my computer.
When I have tried to transfer these songs using the supplied Data Cable to my Sony Eriksson V800, they transfer fine with no problem.
When I try to use them as caller ringtones it wont let me, saying that the DRM key is missing.
What the hell is that and how do I get round it?.
I have tried the same songs on my Sony E K750i with no problem.
I can't for the life of me think what's wrong.
Can anyone help me?
Any help would be much appreciated.

 

It's probably your mobile provider that has locked up the phone so that you have to pay them to download MP3's to use as ringtones. I've had a long running argument with Vodafone about this but still no joy.

One way to get around it is to get your phone 'debranded', which is different from 'unlocking', this reverts your phone back to the factory settings before your provider started locking certain features up. Unlock-a-phone on Sheffield Road will be able to do it for £30-£40, however they wont guarantee that it'll solve your problem.

Hope this helps.

Andy