1) The SIM Application Toolkit APIs, etc, are not part of the SDKs for Symbian phone.
2) While the SIM may have a location for the number, it usually isn't stored on the SIM (or the phone).
The phone number associated with the SIM is not really used for anything, and thus it isn't usually stored anywhere on the phone or SIM.
The operator's network does the mapping of the SIM serial number to a phone number, when it connects a call (if it is allowed) or deliver a message (the calling/sending phone is not involved in that process at all).
In other words, if you're writing an application that depends on knowing what the phone user's phone number is, you have to ask the user to give his/her number somehow.