Check your phone bill asap. There is a scam that when you answer the call you are locked onto a service, which is at premium line rate. I am not saying this is happening to you, and I would doubt it, but I would make a check just in case. Failing that, report these nuisance calls to your telephone service provider, they may assist by putting a trace on the line
Have you checked with the DVLA since these calls have occurred!.