Update (26 March 2014)
The big hush-hush news in Australia is that Google has done a deal with one or two telcos and will be handing out free 1800 numbers for the purpose of giving advertisers the ability to track call conversions in AdWords.
How much will it cost? Nothing – it’s free, as long as you have a Google AdWords account running (even at a trickle).
Will the number change? No, as long as you keep your advertising running with Google, your dedicated 1800 number will remain the same, so you can use it in all your advertising, such as business cards.
How many numbers can I have? Assuming you can have one for every AdWords account, which is usually per website you are advertising. If you have a business with multiple websites, and multiple AdWords accounts, in theory that means you can have a unique free 1800 number for each.