The VoIP.ms wiki says "with VoIP.ms you get free calls from your account to any VoIP.ms DID from US and Canada."
http://wiki.voip.ms/article/Features#Free_Calls_Between_VoIP.ms_CustomersYou can also setup a "Virtual DID" on voip.ms that you can make a call to the OBi using a sip uri address. Voip.ms Virtual DID's have a $0.25 (25 cent) fee per month plus 0.001 (one tenth of a cent) per minute fee for an incoming call.
If you setup subaccounts you can make calls between voip.ms subaccounts via a SIP uri.
You also, of course, can make free calls between two OBi devices using the built-in OBiTalk network.
So to make free calls between the two OBi devices I would select the method I wish to use for the call and setup a simple Speed Dial to make the call.