Ok, i've opened the plastic shield and there's no IMEI print/label inside, but the sticker on the chip confirm me that is a K3765, so probably someone have flashed the wrong firmware because the dongle react to an ATI as an E180.
Reading around, seems that is a ''normal'' behaviour of some stick/phones: if flashed with wrong firmware, result to have a null IMEI.
Strange thing, ive tried to connect to internet (eg, using the stick for the intended use ;), and works, so seems that at least for the data connection, a missing IMEI does not care.
Someone can give me some hint on how to restore the correct firmware? Thanks.