Not an expert here but there are two ways that calls are forwarded. Knowing this may help in specific situations.
Note than when a call is forwarded, the phone doing the forward will usually ring once, telling you that the call was forwarded.
A Conditional call forward seems to be what Google Voice does, ringing a remote phone but allowing the call to be retained by Google so their own VoiceMail can take over if the call is not answered in a certain period of time.
This Conditional procedure is often used in an office when to get a call and want to forward it to another person. You dial their number and stay on the line to see if the person is there, then end the call forward attempt and try a different number.
An UNCONDITIONAL forward is when you force the call t othe remote number and give up your control. Thus the remote phone may be answered by a person, go to voicemail or worse just ring and ring, depending upon the remote phone capability.
Some Voip providers like
Voip.MS and CallCentric have what they call a RING GROUP. A call sent to a ring group will ring all phones in the group until one of them answers or a timeout happens which may send the mto voicemail.
Some providers have a FOLLOW ME service which calls phones in a certain order, maybe 3 rings and if no answer call a second nuber, etc.
On an Obi setup I created, a user sometimes presses the phone button to forward calls uncondionally, using their cellphone as the recipient. The problem with that is when they are away from the physical phone doing the forwarding, they can't undo that until they get home. Also, if the call goes to their cellphone and they don't answer the call, the caller gets a voicemail prompt configured on the cellphone. If the yreturn tha call from their cellphone then the caller gets a message from a telpehone number they did call and may not recognize.