This is a good point, the most foolproof way of (dis)proving that an SMS was sent from some number is to turn to the operator.

Edit: There's the caveat, that even the operator logs can be used only to determine whether a specific handset or mobile subscription was used to send a message. Proving that a person did or didn't send a message is far more difficult.

Actually, on my operator, I can get a listing of all outgoing calls and SMS from my account, with exact times and recipient phone numbers (with few last digits blocked with xxx) just by signing in to their customer service website. Such a listing would not be enough to disprove message content though, if there has been some communication between parties.

I think that by law, operators in Finland must be able to provide a complete breakdown of outbound events on customer's request, since it's the basis for billing. Don't know about other countries though.