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To log into your account to make a top up using a credit card, you are required to supply your own number. This is not printed on the box the usb modem is supplied in or given in any other literature supplied. Furthermore, the information in the supplied leaflets and manuals is incorrect and won't help.
There's a much simpler way. In Vodafone Mobile Connect software, go to the Help menu, then Diagnostics. This will tell you your hardware firmware-version, IMEI, IMSI, and your own number.
This works on my version 9.2 VMC - I would imagine other versions have something similar.
And if that didn't work, you could always send an SMS from your widget to a mobile-phone you have, and get the sender-number off that!