Send a blank TXT message to 521. Vodafone will reply back with the mobile number for the SIM card.
Other useful Vodafone mobile services
- Emergency Prepay Top up: TXT IOU to 468
- Vodafone balance: TXT BAL to 777
- Call me: TXT <mobile number of the person you want to call you> to 798
Source: Vodafone – In case of emergency