Methods and apparatus for use in processing Session Initiation Protocol (SIP) calls in a network environment which includes a circuit-switched (CS) network and an Internet Protocol (IP) multimedia subsystem (IMS) network. In one illustrative technique, a SIP Register message is sent from a mobile communication device to the IMS network for registration of the mobile device. A SIP 200 OK message is received by the mobile device from the IMS network in response to sending the SIP Register message. The SIP 200 OK message has one or more variable call parameters or a network address at which to obtain the variable call parameters. The variable call parameters may include an E.164 number which may be dynamically assigned to the mobile device by the IMS network, and/or a time or timer value which defines a time period for which the E.164 number remains assigned to the mobile device. Other information may be included such as preferred access network/technology information. The variable call parameters are stored in memory of the mobile device and utilized for processing each one of a plurality of SIP calls involving the mobile device. After registration, the mobile device may initialize a timer with the timer value, run the timer and, when processing a SIP call, cause a CS call setup message which includes the E.164 number to be sent to the IMS network for routing of the call if the timer has not yet expired. If the timer has expired, the mobile device may refrain from utilizing the deassigned E.164 number in the CS call setup message and alternatively obtain and utilize a new E.164 number or an altogether different technique for processing of the SIP call. Alternative techniques for obtaining parameters and formatting the data are also described.