We had been receiving mysterious inbound calls from 3 digit callerid numbers: 100 & 101. The router was not set on DMZ mode and there was no port forwarding. I wondered if these calls were coming in via the OBi network, but ultimately it didn't matter.
I have found the best way to prevent robot calls is to restrict which IP addresses that can connect to my OBi.
Voice Services -> ITSP Profile A -> SIP -> X_AccessList : voip.ms_ip_address
Then, only Voip.ms will be permitted in. One thing to watch out here: if a voip.ms server goes down, they might reroute calls via a different server. So you should add a comma-separated list of all the voip.ms server IP addresses for greatest reliability.
My current list:
209.62.1.2,74.54.54.178,67.215.241.250,74.63.41.218,78.129.153.20,67.205.74.164,174.137.63.206,69.147.236.82,68.233.226.97,64.120.22.242,174.34.146.162,174.142.75.171,174.137.63.206
