PBX Version: X.13.0.15
Linkus iOS App: 2.9.X or later
Linkus iOS client can't receive a call when the Linkus APP is running on the background after the App or PBX upgrade.
In a recent firmware version of X.13.0.15, we have upgraded our call notification push mechanism. To make it work properly, it requires a binding relationship between the PBX version and Linkus iOS app version. If only one of them is not the latest version, it might cause the issue of Linkus iOS client can't receive call issue.
You can try to upgrade the S-series PBX to the version 18.104.22.168(or later) and also upgrade the Linkus client to 2.9.x(or later).
After the updates, you might still need to wait for 1-2 days due to the Apple Notification Push server requires time to refresh the push token. After the period, the call notification push would start to work again.