The following is a guest post by my friend and colleague, Johnny Mastin.

Ok, so this post isn't Apple-specific but it can be used with Apple products. 🙂 A little while back Google opened up Google Voice (GV) to everyone. Prior to that, you had, to be part of the technocrati or be invited by the technocrati. This post is to talk about some of the features of GV. What is GV?

It is a telephone service from Google. US-based users can get a US phone number. You set up your GV number to forward to any of your configured phones (cell, home, work, etc). You also have the ability to place and receive calls via Google Talk. How you answer the call is very flexible. You can choose to call a specific number during the day vs. evening. You also have the ability to customize your voicemail message depending upon who is calling. All voicemails are sent to you as an audio bite as well as transcribed and sent via email or SMS. You also have the ability to send calls directly to voicemail or even block certain callers. Additional features are the ability to switch phones in the middle of a call, record calls and have ad-hoc conference calls. More details can be found at http://www.google.com/voice (definitely watch the videos).

So, anyone with a Google account can get a GV number. Your GV number can then be set up to forward to any or all of your phone numbers. When someone calls your GV number, the phone you've setup starts ringing. Outgoing calls though GV shows up with your GV number on the caller id. You can even set it up so that you can make and receive calls via Google Talk. One of my co-workers was on a conference call using his iPhone headset plugged into his MBP. You couldn't tell he was using GV via Google Talk on his computer.

Can you use it with your iPhone? Yes. And with Apple's recent reversal on their stance for apps, expect to see some GV apps (2 just were approved and released this week). You can use it on any cell phone (even landlines too!). If you call your GV number from one of your listed phone numbers, you have the ability to type in your PIN, press the number 2 and then dial another number. That second number you dial will be the one dialled using your GV number as the caller id. If you use Google's method available through the Google App, you are sent to a webpage that looks like a dialer. Android and Blackberry users get an app. The jury is still out whether or not Google will resubmit their Google Voice app to the AppStore. The webpage app has access to your Google contacts. When you dial through that webpage app, it will ask if it is ok to use your cell phone to dial an access number. Click yes and it will dial a GV access number which will then dial your party with your GV number. Keep in mind BOTH of these methods will count against your cell phone minutes! (There is no free lunch) Calls made through your GV number are free to the US and Canada for the rest of 2010. International calls are very cheap. In many cases, like $.02/minute. My feeling is that Google will keep it free come 2011 or charge a very basic fee (like Skype) for unlimited calling to US and Canada.

So, where does all this lead? Here are a couple of potential use cases:

• If you're someone who is in IT support. I've been in some sort of support function for most of my career. That included carrying a pager or a cell phone. I despise the Batman utility belt look. I really don't like 50 things hooked up to my belt. It might be geek chic, but it isn't me. GV to the rescue. Give the ops center, boss, whoever the GV number. Set it up to forward to whatever phone you want at whatever time. One of the things at a previous employer that was a royal pain was having to give the ops staff all of my numbers and they had to play phone MarcoPolo to find me. Give one number and be sure you are at the other end of the line (be it at work from 9-5, then the cell, then home, or cell all the time or whatever). Also, if/when you change jobs, it is easy to block that caller or set up a special voicemail for them saying you no longer work there rather than having to change all of your phone numbers.

• If you're part of a group that has an on-call rotation. Nothing like having a hot pager or phone you have to pass around. (See the Batman look above) It is even harder to do if your group is spread out in different locations. Use the GV number and just forward that number to the person who is 'it' for the week. The ops center then has 1 number they have to call to always get the right person who is on-call. Setup 2 GV numbers if you have a primary and secondary on-call rotation.

• If you're at the club and you're giving away the digits. Remember the cute member of the opposite sex you met who later turned into someone you didn't like? Well, blocking them is a lot easier than getting a new cell number.

• You're a student away at school. This is a good way to keep in touch without having to have a cell phone. I remember when my sister went away to college about 10 years ago. Cell phones and plans were notoriously expensive. We tried using voice chat and it was god awful at that time. This is a lot better.

• In my current role, I talk with a lot of vendors. Some of them have my cell phone (just the nature of my gig). Using GV makes it easier for me to be portable. In the office, out of office, using up my minutes, etc. Also, I've also seen it where a customer had inadvertently found out a co-worker's number and was calling them directly for support. It can be a little annoying when customers should be dealing with ops support first. It is easy to block them with GV.

Ok, maybe only certain use cases might be applicable to you. In any case, GV can be very useful. Remember the example of my coworker who used it for a conference call? Well, by using GV and a broadband connection (from a cafe), he was able to attend a conference call without tying up his cell phone and chewing up minutes. Pretty slick.

Another good part of GV? It's free.