We have recently deployed a new 3CX system for one of our clients and ran into an issue with the Windows client. The client, when attempting to register an extension on the new 3CX system will state "This client is not compatible with the current version of 3CX Phone System. 3CX Phone System Version 15 updated with the latest service packs is required". However, we are running version 15.5.5932.2 on both the newly deployed system as well as a test system in our office and the client works fine on our test system. We're also running the latest version of the Windows client. So here's what we know so far in regards to the major differences between our production deployment and our test setup in our office: 1. We changed the USER AGENT in the 3CX Parameters on our production system because: (1) Our SIP trunk provider blocks "3CXPhone" due to toll fraud issues and (2) it's recommended by 3CX anyway to help mask your PBX's identity. 2. Our test setup in the office has the default USER AGENT (i.e. we didn't change it) 3. When we change the USER AGENT to something other than than the default on our test setup, the client fails and generates the same error as our production system. This indicates to us that there's an issue with the Windows client checking the USER AGENT on the 3CX PBX Server and failing because it thinks the client is out of date. 4. We changed the USER AGENT back on our test system, restarted the SIP services and the client would connect without issue. 5. We've verified with our SIP Trunk provider that they are indeed blocking the USER AGENT "3CXPhone" and they can't unblock it specifically for our connection, nor should they have to. Has anyone else run into this or is there another setting that we're missing? Thanks in advance for any insight on this issue.