You can start by calling your mailbox on the internet, our mobile applications, or use their own Google Voice number from one of your phone and select option 2 to the call.

Keep in mind that Google Voice is only available in the U.S., you need a working phone in the U.S. to Google Voice account.

To get a google voice invitation go to https://services.google.com/fb/forms/googlevoiceinvite/

here is how invitation mail  will look like 

A personal phone number that rings all your existing phones when people call
All your voicemail from a mailbox with unlimited storage online, and free voicemail transcripts to your phone and e-mail
Low-priced international calls to more than 200 countries and free SMS
Other powerful features, the first phone spam filter to protect against unwanted callers, ability ListenInTM to your voicemail, while the left, conference calling and more

Call screening – Announce and screen callers
Listen in – Listen before taking a call
Block calls – Keep unwanted callers at bay
SMS – Send, receive, and store SMS
Place calls – Call free within the continental US and to Canada
Taking calls – Answer on any of your phones
Phone routing – Phones ring based on who calls
Forwarding phones – Add phones and decide which ring

Voicemail as easy as email, with transcripts
Voicemail transcripts – Read what your voicemail says
Listen to voicemail – Check online or from your phone
Notifications – Receive voicemails via email or SMS
Personalize greeting – Vary greetings by caller
Share voicemail – Forward or download voicemails

More cool things you can do with Google Voice
Conference calling – Join people into a single call
Call record – Record calls and store them online
Call switch – Switch phones during a call
Mobile apps – Try our apps for Blackberry and Android phones
Mobile site – View your inbox from your mobile
GOOG-411 – Check directory assistance
Manage groups – Set preferences by group