A lot of mobile network operators wash this information about or have it hashed into some other form (which means it can still be used as a unique identifier)
Some popular headers to check
X-UP-CALLING-LINE-I
X_NOKIA_MSISDN
X_H3G_MSISDN
MSISDN
X_MSISDN
X_NETWORK_INFO
X-WAP-MSISDN
X-UP-SUBNO