Contact the ISP and hope that they disclose this information, or even store it at all.
Freeserve etc. doesnt log callers numbers and the IP addresses they receive - perhaps they can tell you which account had the IP address at the time, but with all the free ISPs you can make absolutely fake accounts as they never check anyway.
Even if the provider is aol, the first month (ie pre billing) you get people using false names and addresses to abuse the free trial period.