Because of the nature of voip you can't just pickup the phone and join the call but I would think you should be able to setup a 3-way call. You should be able to dial the other extension by using the
voip.ms internal extension number (101 102 etc). Review the 3-way calling instructions in the OBi Administration Guide and also in you Yealink ip phone manual.
Voip.ms says that you can call between sub accounts using the internal extension number if the subaccounts are registered to the same
VoIP.ms server.