ok first of all, do not worry... this is no biggie, seriously!!!
i know you are glad to hear this so listen carefully...
first of all... ISPs never do anything about these letters... i got one before and they told me the same thing... and i did nothing about it and it blew over... no one cares...
but in your case since they are requesting a phone call i suggest you call them and tell them that you have no idea that anyone downloaded something illegal... they cannot prove it was necessarily you...
you can claim that you have a wireless router without a lock (WEP for example) because you are just not tech savy... what are they going to say to that... i know... nothing.... they have no proof...
really give them a call and you'll see that it was really nothing... and if you want to prevent these letters in the future download peerguardian2
google it... it blocks certain IP addresses when you are torrenting or downloading... once i started using peerguardian2 i never got a letter again.
Thanks my mom is going to say a minor (myself) did it. Hopefully they don't do anything. I am scared shitless.
hope that clears a few things for you and relieves your worries... they really do not have any proof on you... delete the file and play stupid... remember... you have no idea how it happened