SMS retention periods

Hi guys, just wondering if anyone can shed some light on the following question I have been trying to work out.

Do network service providers (vodafone, O2 etc) maintain records of the contents of SMS in the UK or is it just sender/reciver deatails? and if so, how long?

I was under the impression that SMSC's only maintain records whilst trying to deliver the message, after which they are gone and records of their content cannot be recovered

Thanks