A first communication provider supports an IP multimedia subsystem (IMS) having a border session control function (BGCF) that receives SIP requests. The SIP requests correspond to different SIP methods and specify various feature tags. The feature tags correspond to available services or media types that may be available from the IMS of a second communication provider. When receiving a SIP request that addresses a user supported by the second communication provider, the BGCF of the first communication provider checks the method and feature tag of the SIP request to make sure that they are supported by IMS of the second communication provider. If they are not, the SIP is rejected and a failure message is returned. The BGCF may also modify certain parameters of the SIP request, such as by removing one or more offered codecs or preconditions.