Method and System for Enforcing Proxy Use Within an Enterprise Communications System

1. A method of enforcing use of an enterprise voice application server as a SIP proxy server for enterprise VoIP communications, wherein the enterprise includes a client device having a first IP address and a client uniform resource identifier (URI), and a Private Branch eXchange (PBX) configured as a SIP registrar having a registration entry binding the client URI to the first IP address, the enterprise voice application server having a proxy URI and a second IP address, the method being performed by the client device and comprising: receiving, at the client device, a SIP request message directly from the PBX addressed to the client URI for a call made to the client device; and determining, from the client device, that the SIP request message was not sent via the enterprise voice application server and, upon said determining, then: sending, from the client device, a SIP 305 Use Proxy response message to the PBX, wherein the SIP 305 Use Proxy response message references the enterprise voice application server in a Contact field, and receiving, at the client device, the SIP request message from the PBX routed via the enterprise voice application server, wherein the enterprise voice application server is the SIP proxy server for a remaining SIP dialogue for the call.

2. The method claimed in claim 1 , wherein the SIP request message is an INVITE message relating to a call request from a remote party received by the PBX over the public switched telephone network.

3. The method claimed in claim 1 , further including sending a SIP REGISTER request to the enterprise voice application server and receiving a SIP 200 OK response message.

4. The method claimed in claim 3 , further including forwarding the SIP REGISTER request from the enterprise voice application server to the PBX, and creating the registration entry.

5. The method claimed in claim 1 , wherein the PBX is configured to refuse a registration request that results in binding more than one URI to the same Contact address.

6. The method claimed in claim 1 , wherein the determining comprises reading a Via header of the SIP request message and determining that an address for the enterprise voice application server is not listed in the Via header.

7. The method claimed in claim 1 , wherein the client device comprises one of a mobile device, softphone, and desk telephone set.

8. The method claimed in claim 1 , wherein the SIP 305 Use Proxy response message includes the Proxy URI in the Contact field.

9. The method claimed in claim 1 , wherein the SIP 305 Use Proxy response message includes the second IP address in the contact field.

10. A client device associated with an enterprise, the enterprise including a Private Branch eXchange (PBX) configured as a SIP registrar and an enterprise voice application server having a proxy uniform resource identifier (URI) and a second IP address, the client device comprising: a communications subsystem for engaging in IP-based communications with the enterprise, wherein the client device is configured to be assigned a first IP address and a client URI; a memory; a user interface for outputting information and for receiving user input; a processor for controlling the communications subsystem, the memory, and the user interface; and a communication application executable by the processor and configured to receive a SIP request message directly from the PBX addressed to the client URI for a call made to the client device, wherein the SIP registrar includes a registration entry binding the client URI to the first IP address, and wherein the communication application is configured to determine that the SIP request message was not sent via the enterprise voice application server, and wherein the communication application is configured to, upon said determination, then: send a SIP 305 Use Proxy response message to the PBX when the SIP request message was not sent via the enterprise voice application server, wherein the SIP 305 Use Proxy response message references the enterprise voice application server in a Contact field, wherein the enterprise voice application server is the SIP proxy server for a remaining SIP dialogue for the call.

11. The client device claimed in claim 10 , wherein the SIP request message is an INVITE message relating to a call request from a remote party received by the PBX over the public switched telephone network.

12. The client device claimed in claim 10 , wherein the communication application is configured to read a Via header of the SIP request message and evaluate whether an address for the enterprise voice application server is listed in the Via header to determine whether the SIP request message was sent via the enterprise voice application server.

13. The client device claimed in claim 10 , wherein the client device comprises one of a mobile device, softphone, and desk telephone set.

14. The client device claimed in claim 10 , wherein the SIP 305 Use Proxy response message includes the Proxy URI in the Contact field.

15. The client device claimed in claim 10 , wherein the SIP 305 Use Proxy response message includes the second IP address in the contact field.

16. A non-transitory machine-readable medium having encoded thereon computer-executable instructions for enforcing use of an enterprise voice application server as a SIP proxy server for enterprise VoIP communications, wherein the enterprise includes a client device having a first IP address and a client uniform resource identifier (URI), and a Private Branch eXchange (PBX) configured as a SIP registrar having a registration entry binding the client URI to the first IP address, the enterprise voice application server having a proxy URI and a second IP address, the computer-executable instructions comprising: instructions for receiving, at the client device, a SIP request message directly from the PBX addressed to the client URI for a call made to the client device; instructions for determining, from the client device, that the SIP request message was not sent via the enterprise voice application server; and instructions for, upon said determining, then: sending, from the client device, a SIP 305 Use Proxy response message to the PBX when the SIP request message was not sent via the enterprise voice application server, wherein the SIP 305 Use Proxy response message references the enterprise voice application server in a Contact field, wherein the enterprise voice application server is the SIP proxy server for a remaining SIP dialogue for the call.