Examine your provider call detail records and your OBi Status - Call History to determine the nature/source/route of the calls, if possible.
Who is your provider?
Disable OBi services not being used:
Voice Services
SPn Service - SPn Service::Enabled = NOT checked
Configure OBi services to prohibit anonymous SIP, if supported by the ITSP:
Voice Services
SPn Service - SIP Credentials::X_EnforceRequestUserID = checked
Configure OBi services to prohibit anonymous SIP when above X_EnforceRequestUserID is not supported by the ITSP:
Voice Services
SP1 Service - SP1 Service::X_InboundCallRoute = {>('AuthUserName'):ph1} ;route inbound to PH1 except prohibit anonymous SIP (Oleg method used prior to X_EnforceRequestUserID); does not work for Builds 4303 and 4318; can omit (' ') if AuthUserName is all digits/numbers.
Most recent firmware supports Taoman's suggestion:
Voice Services>SPx Service>X_AcceptSipFromRegistrarOnly
And then you should not need those ugly inbound call route bits that try to route every possible badness to nowhere.
OE