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: