How strange!
The first thing would be a soft reboot of the device—hold volume down and power until the device restarts.
Still nothing? Since it says "on another call," I'm assuming this means a setting isn't enabled to block calls.
Are you able to test another SIM in your phone—same result? And try your SIM in another phone—same result?
If your SIM in another device does the same, then I would contact your provider.
If another SIM does the same on your device, I would contact customer support. The only thing I'm seeing online to fix it in that case is a factory reset.
