To elaborate on RonR's comments:
If Call Waiting is disabled and you are on a call, the OBi will present "busy" status to GV or
VoIP.ms, or "no answer" status to a line or device connected to the LINE port. On GV, if Google Chat is the only forwarding phone selected, the caller will immediately get voicemail. If there are one or more other phones, they will ring normally; if unanswered the caller will get voicemail after 25 seconds. On
VoIP.ms, the caller would normally get voicemail immediately, but if you have a ring group or other fancy routing selected, your other phones could ring. A landline call would normally be ring-no-answer, but could be picked up by landline's voicemail, or by an answering machine or other phone connected in parallel with the LINE port.
If Call Waiting is enabled and you ignore the beeps, it will beep again after about 10 seconds. If you ignore that, too, after another 15 seconds (or as programmed for the service involved), the caller will get voicemail.
Some IP phones, which could be connected to GV through the OBi, or directly to
VoIP.ms, will show "answer" and "reject" softkeys on call waiting, just like a cell phone. If you press reject, the caller will immediately get voicemail.
You could connect additional ATAs and/or IP phones, such that when you are on a call, the other phones would ring and could be answered by another person at home. The two concurrent calls could be on the same service or different services (though obviously only one call on a landline).