Random inbound calls with no audio.
Q. I keep getting random calls from numbers like "100", "1000" and "Sipvicious", but no one is there when I answer. What's going on?
A. These are most likely "spam" calls which are made directly to your SIP phone over the Internet. These calls do not come from our system / network, but from scanners on the Internet targeting blocks of IP addresses at random. Luckily it is possible to overcome by changing a setting on your handset.
You will need login to the handset's web interface, by browsing to it's IP address on your local network.
If you are using Grandstream handsets you can enable the setting "Check SIP User ID for Incoming INVITE" and "Accept Incoming SIP from Proxy Only" under 'Accounts' > 'Account 1' > 'SIP Settings' > 'Security Settings'. On the older version of the firmware this setting is just under 'Accounts' > 'Account 1'. You will need to save and apply the setting.
If you are using other makes of SIP handsets then you can change the local SIP port setting from '5060' to something else, for example '5868', and this will usually do the trick.