I have one google voice number ("A") that forwards to two mobile phone lines ("B" and "C"). Voicemails left for all three numbers (A, B, or C) are collected in my single google voice inbox. When viewing a message, how can I determine which number was dialed?
I've searched high and low because I had a similar question. Unfortunately, I've been unable to find a positive answer. It appears that, right now, it's not possible.
You service provider (the telephone company) is certainly forwarding the dialed number, but Google isn't displaying to you. The dialed number is known as "DNIS" (Dialed Number Information Service). It's not a new technology or concept; answering services, and businesses that use PRI and SIP trunks use DNIS to route incoming calls to the correct departments, etc.
I wish I knew someone at Google to ask: "Do you capture the DNIS information and are there any plans to make it useful?"
As Nathan indicated, there isn't any way to see this in your GV inbox, but you can often determine which number they called with a bit of sleuthing.
Assuming your two cell-phones are powered on and in range, you can look at the call history of the phones themselves and if you see the caller in the missed calls section, they called that number (or the GV#, if they show up on both phones).
Alternately, in Google Voice settings, under Calls, you can set the incoming caller id on your cell-phones for calls to your GV # to be the GV # itself - this makes it more obvious that someone called the GV # (although you then have to do the correlation using just the time, since the original caller-id information will not be on your phones - although it will still be in your GV call history).