A server for forwarding a SIP, Session Initiation Protocol, message, the server comprising: a processor; and a computer program product storing instructions that, when executed by the processor, causes the server to: receive a WebSocket handshake request from a client device (2a-b), the WebSocket handshake request comprising a SIP service capability indicator for the client device (2a-b), wherein the SIP service capability indicator comprises two or more headers of the WebSocket handshake request, each header indicating a single SIP service capability of the client device (2a-b); establish a WebSocket connection; store, in a data memory, service capability data for the client device (2a-b), the service capability data being based on the SIP service capability indicator; receive the SIP message from the client device (2a-b); add a header field to the SIP message, the content of the header field being based on the service capability data, resulting in a modified SIP message; and forward the modified SIP message to a SIP application router, wherein the SIP application router parses the added header field and forwards the modified SIP message to an appropriate SIP application. a processor; and a computer program product storing instructions that, when executed by the processor, causes the server to: receive a WebSocket handshake request from a client device (2a-b), the WebSocket handshake request comprising a SIP service capability indicator for the client device (2a-b), wherein the SIP service capability indicator comprises two or more headers of the WebSocket handshake request, each header indicating a single SIP service capability of the client device (2a-b); establish a WebSocket connection; store, in a data memory, service capability data for the client device (2a-b), the service capability data being based on the SIP service capability indicator; receive the SIP message from the client device (2a-b); add a header field to the SIP message, the content of the header field being based on the service capability data, resulting in a modified SIP message; and forward the modified SIP message to a SIP application router, wherein the SIP application router parses the added header field and forwards the modified SIP message to an appropriate SIP application. receive a WebSocket handshake request from a client device (2a-b), the WebSocket handshake request comprising a SIP service capability indicator for the client device (2a-b), wherein the SIP service capability indicator comprises two or more headers of the WebSocket handshake request, each header indicating a single SIP service capability of the client device (2a-b); establish a WebSocket connection; store, in a data memory, service capability data for the client device (2a-b), the service capability data being based on the SIP service capability indicator; receive the SIP message from the client device (2a-b); add a header field to the SIP message, the content of the header field being based on the service capability data, resulting in a modified SIP message; and forward the modified SIP message to a SIP application router, wherein the SIP application router parses the added header field and forwards the modified SIP message to an appropriate SIP application.