The call pop up alert feature works when a phone that has the call pop up set up on that line rings. 

If you are in a hunt group - the call pop up will only be displayed when the associated phone line rings your phone. 

For instance it needs to ring on line A to show up on the screen that user has and then it will jump to line B to show up on the screen that person uses.

Below is an example of the call pop up alert on the screen:

If we altered an incoming line to ring all phones at once (in a group ring) then the call pop up alert would be displayed for all users set up in the group - whereby the alert would display on the screens for all users linked to the phones simultaneously.