When I call Vodafone and they require the 20 digit number on the sim card ,it requires that I remove the back of the phone, then the battery and then remove the SIM card surely this disconnects the call (I have not called via my phone as I use CHAT) I would have thought that with todays technology the phone would be able to register the SIM card without all the fuss of removing the back ,taking out the battery and then trying to remove the SIM card and if you had short stubby fingers like me it is a hell of a job ,of course I bet some Smarties make a note of their SIM cards I didn't and finally If I drop my phone the back falls of battery comes out but the bl$£dy SIM card does not budge.
I've never had to give this number to customer services. Who is your provider and why are they asking for that? The number on your sim will be associated with your phone number - surely they only need that.