Another screen name radonly signed on thru my t-zones.

I picked up my cell phone today and aother s/n I've never heard of was signed on and a conversation was had. Could it be an AIM/T-Mobile error? Or did someone use it when I wasn't around? Have you ever heard of the happening before?

I have a Nokia 3650 & it was signed on through t-mobile's t-zones.
Also, is there anyway, other then looking up the profile, that I can trace this sn.