Google Voice testing latency to POTS (plain old telephone system), GSM, CDMA and a VoIP provider, because I couldn't find much information other than conjecture about actual voice latency on these systems.
I used Audacity to do the tests, it's really easy to do if you have a mic and speakers. (I'd be interested to hear what others measure. Be sure to calibrate audacity so you don't include the latency of your sound card in the measurements.)
These tests measure the time it takes, for sound to enter one device and come out the other device. In other words, the one-way time it takes for your speech to reach your calling party (and vice versa). They say that just 40 or 50ms of latency is the point at which voice delay becomes noticeable.
Here are the results!
POTS to CDMA: 149ms
POTS to GSM: 162ms
CDMA to GSM: 291ms
POTS to Google Voice to CDMA: 279ms
POTS to VOIP: 326ms
CDMA to VOIP: 450ms
VOIP to GV to CDMA: 750ms
From these, I estimate POTS has a latency around just 10ms...extremely good, and as to be expected. Once you get on a cell-to-cell call, you have almost 300ms latency. At this point conversation starts to becomes more half-duplex than full-duplex; that is, talking over one another and waiting to speak, similar to a walkie-talkie.
Google Voice seems to be like a cell phone in terms of latency, so calling Landline -> GV -> Cell is similar to a cell-to-cell call. Cell to GV to cell should be something like 430ms, quite a bit of delay.
CDMA may be slightly better than GSM, not really significantly different.
VoIP is twice as worse than a cell phone, you'd be far better off using a cell phone with a home phone adapter like Xlink. I really tried hard to get the lowest lag on VoIP too, by using 0.010 ms packet size and lowest possible jitter buffer. Ping was 50ms to SIP/RTP servers.
Google Voice does 3 things, SMS, Voicemail, and phone calls. The other features are superb. But in my opinion, the added latency for phone calls is not worth it. Calls are delayed enough with cell phones; it doesn't make sense to add more frustration when making phone calls even for the convenience of a single number.
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