Dear Aunt TUAW,
It's called latency, and it happens with all kinds of computer telephony solutions like Google Voice and Skype, as well as with iOS voice chat apps. What it means is that you're talking in their past. It takes time for your voice signal to make its way to the other party, and theirs to you. Historically, this was mostly an issue for intercontinental or satellite calls, but the march of technology has delivered it to calls you make across town as well.
The lag in calls can vary from mere milliseconds up to a second or more, in bad cases. Google Voice in particular seems to have trouble with latency; traditional VoIP services may do better (assuming nothing's wrong with your network connection). By comparison, plain old telephone service has latency of about 45 milliseconds.
You can find out exactly how much lag you're dealing with by using a simple trick. Ask the person at the other end to mirror you as you slowly count to 10. Tell him or her to count along at the same time. You'll hear the differential between your numbers and theirs, to get a sense of how far apart you are.
There are ways you can deal with the latency problem with VoIP. Here are a few suggestions that might be able to help.
Identify Let your partner know that VoIP is in use as early in the conversation as possible, and that there may be lag involved.
Switch to Wired Using a Wi-Fi connection can exacerbate latency. Try plugging in directly and see if that helps. You may also want to test a call while connecting your computer directly to your cable or DSL modem to see if your router is making it worse; most router manufacturers have specific configuration tips for handling VoIP traffic.
Avoid Active Listening Many people show they are paying attention by interjecting feedback into a conversation like "yes" and "I know!" VoIP disrupts the normal flow of conversation by introducing an unnatural rhythm. If you're saying "yes" or "I know" while someone is speaking, rather than in natural pauses, your conversation is going to stumble over itself.
Proactively set VoIP-aware talk patterns Using phrases like "go ahead" and "Okay, now you..." help transition between one speaker and the other, creating a smoother discussion pattern in a latency-burdened channel. Also, don't be afraid to monologue to get your point across -- normal back and forth discussion patterns are at their weakest with VoIP.
Offer alternatives It's fine to say "Can I call you back on my cell?" when latency significantly hinders communication.
So, that's what Auntie has to suggest. Surely she has missed a few obvious suggestions. Add yours in the comments!