A system and method to enable mobile devices to access non-IMS application services over an IP Multimedia Subsystem (IMS) is described herein. In order to access a non-IMS application service via the IMS, messages in a format that are suitable for exchange with the non-IMS application service are encapsulated by the mobile device user agent into SIP messages. The SIP messages are then routed by the IMS to a services gateway. The services gateway extracts the application service messages from the SIP messages and provides the extracted messages to the appropriate application service. In this fashion, the mobile device is able to request services from an application service via SIP messaging, even if the requested application service does not support SIP messaging.