Phantom calls from unknown numbers (100, 1000, 101, 1, 1005 ...) are usually from SIP scanners/ scammers and are called "ghost calls".

If you are receiving "Ghost calls", usually they are being sent to you by SIP Scanners for scamming purposes.

You could try to block port 5060 except for the IP of your server with your firewall, or maybe easier change the Zoiper SIP port in ZoiperH5 -> Settings -> Advanced -> Network -> SIP options sub-tab (or from the advanced network configuration menu of your account depending on the product that you use) settings menu to something else than 5060.

You can find some additional information here.