If the original software is on the phone, enter *#*#7378423#*#* in the dialler, then look in Service Info, Software Info for the customisation version. Once you have it, look in the list here to see which network the phone was provided by - http://forum.xda-developers.com/show....php?t=2315914
If that doesn't work, ask the person you got the phone from or contact Sony, as I believe that there's no way to tell from the IMEI without access to Sony's database.
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