You said you use GV handling voicemail for your cellphone - as do I. I said nothing about VOIPo simulring.

Assuming that you have GV ringing your cellphone and your VOIPo number (as I do), the default behavior, if someone calls your cell number directly but you don't answer, is for GV to then ring all your other GV phones. You then run the risk of VOIPo picking up and taking the message instead of GV. I gather you don't want this.

There are two solutions, actually. The first is the one I presented - where you tell GV to NOT ring your other phones if your mobile does not get answered. The other is to set "call presentation" in GV where, if you pick up, you must then press 1 to be connected.

As far as I know, there is no reasonable way to have GV handle voicemail for your VOIPo line. This has been requested.