The only ways I have found are:
1. You can send a text from the router if it is registered to the network by using the "router#cellular 0 gsm sms send 07xxxxxxx test"
2. Some providers put the number on the card
3. You can give the SIM number to your provider and they will tell you the mobile number
4. Insert the card into a PDA or mobile phone and call a phone with number display.