In order to monitor VoIP between two locations, it is necessary to specify the links that you want to monitor using OpManager, i.e, you will have to define the source and target. The source device (Cisco device with IP SLA agent enabled) must be discovered in OpManager and the target/destination device should also be an IP SLA responder enabled Cisco device.
Please follow these steps to discover a source device in OpManager.
Once the device discovery is completed, cross-check the Vendor name. The Vendor should be 'Cisco' and not 'Cisco Systems'.
For more information on monitoring credentials: https://www.manageengine.com/network-monitoring/help/add-credentials.html