Why is my agent not able to see the incoming call pop-up?

You should verify the Inbound Group configuration related to the agent and check if the Start Call and Dispo Call URLs are configured in the group.

To enable incoming call logs and incoming pop-ups in Vtiger, Vtiger URLs need to be configured in Inbound groups. 
Follow these steps to enable incoming call logs and pop-ups:

1. Log in to the VICIdial Admin panel.
2. Go to Inbound Groups.
3. Click Modify under any one active group.
4. Modify the following fields:
   1. Start Call URL - Copy the Start Call URL generated in Vtiger from -> CRM Settings -> Phone Configuration
   2. Dispo Call URL - Copy the Dispo Call URL generated in Vtiger from -> CRM Settings -> Phone Configuration
5. Click Submit to save the changes.

To learn more about VICIdial integration, click here.