You would need to speak to customer services in regards to this, for them to process some kind of request to the relevant department. Or, for them to give you advice on the next step. If it is a legal issue, you may need to contact the police beforehand, as this could be an issue which either the Nuisance calls department or Police liaison deal with.
The above is just my own recomendation, so dont take it has fact.
Your first point of call will be by calling 20202 on your mobile, and asking for assistance.
If you're talking about text messages to your phone, if you have deleted them without backing them up, I don't believe there is any way to retrieve them.
There are a number of ways of retrieving data when needed, i do know that, but to what extent and to what timescale i don't know.
But anything is worth a shot these days