OS - Windows 10 Outlook - 2016 3CX softphone windows - 15.5.1694.0 The integration was working fine then all of sudden it just stopped working. Keeps coming up with an error. Customized functionality in this application will not work because the certificate used to sign the deployment manifest for 3CXOutlookAddin or its location is not trusted. It is just only popping up on one computer. Steps reinstalled 3CX software. Deleted all the 3CX folders and files in \programdata and in users\appdata\roaming Checked Firewall just incase. Even that looks good. Not sure what is not trusting that certificate for 3CX inside outlook. Thanks.