A ghost call is a call in which, when the recipient of the call answers, there is no one on the other end of the call, or you could receive calls from random (short) phone numbers with no one on the other line when you answer.

Log into the phone's web interface (using an IP number).

Select 'Settings' and then select 'SIP'

Change 'Local Sip Port' and 'TLS SIP Port': change from, e.g. 5060 to 5160 and 5061 to 5161.

Set your firewall to be more restrictive. Our advice is to permit all SIP / VoIP traffic from our platform. The IP range you should use is 195.35.114.0/23 (or 195.35.114.0/24 and 195.35.115.0/24). All other SIP / VoIP traffic you can block.

Yealink support page

Some information from the Yealink page

If it's a Yealink phone (there quite prone to receiving ghost calls) go to the:

• Features tab and General information
• Set Accept SIP Trust Server Only to 'Enabled' and 
• Allow IP Call to ‘Disabled’
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Accept SIP trust server only allows the phones to only accept the SIP message from your SIP server and outbound proxy server. It can prevent the phone from receiving fake (ghost) calls with phone numbers like 100, 1000, and so on. If you enable this feature, the phone cannot accept an IP address call.