If you own a Nokia S60 based cell phone with built-in WiFi (such as the N95, E71, etc), you may be able to get free worldwide calling to/from any US number just by connecting to a wireless hotspot. This service works by combining the phone number forwarding and management service GrandCentral with the SIP internet telephone service Gizmo5. It is extremely simple to set up, though there are a few issues upfront.
First, GrandCentral seems to have stopped giving out invites for their service. Because it is currently in beta, there's no telling when signups will be available again. For now, I'd recommend reserving a number on their main page. If you already have a GrandCentral account from a random forgotten night when you came across an invite, signed up, told yourself you would use the number as your professional work line, then almost entirely forgot about it, you're set.
Second, the settings on compatible Nokia phones may not be exactly the same. What worked for me on my E71 will probably work on similar phones, but the process might not be identical. There is a definite degree of flexibility in this tutorial.
That being said, here is how you go about setting up all these services:
1) Sign up for a GrandCentral (if possible) and Gizmo5 account. The Gizmo5 sign up procedure is fairly self explanatory. After you create an account, you'll be presented with your SIP number. Though it may look like a normal phone number, "real" phones cannot actually call it. Keep this number handy, as you'll use it in the next step.
2) Sign in to GrandCentral, go to the "Settings" tab, then click on "Add/Edit Number" in the Phones category. In the drop-down menu, select "Gizmo" as the phone type, then input your SIP number from the previous step in the applicable box. Next, name this phone, and press save. Finally, make sure the check box for "All Callers" is selected under your newly added Gizmo phone.
3) Now that you've linked your GrandCentral number to your Gizmo5 account, the next step is to set up Gizmo on your Nokia phone. Navigate to the "Connectivity" folder on your S60 device, and select "Internet tel." The screenshots below show the process on my old N95. Here's where things may be inconsistent. On my E71, the "Discovering services" dialogue would lead to an icon directly for setting up a Gizmo Project number. From there, you simply enter your account information, select whether or not you want it to use network data, and you're set. On my N95, however, this icon never appeared. It might have been a mere fluke, as I see no reason as to why this handy setup application wouldn't appear on all compatible phones. If this is the case, the manual instructions posted here should suffice. (Note: In the manual instructions, all the smilies are actually :p, as in "3.Proxy Server: sip:proxy01.sipphone.com")
4) Once Gizmo is successfully configured on your device, you can now receive any call dialed to your GrandCentral number absolutely free. If you want to dial out of your phone to US numbers for free, this requires a little more work.
One of the great features of GrandCentral is its built in address book. Within this address book, you can click on a "Call" button next to a contact, which will make GrandCentral call your phone (in this case, your Gizmo number), then it will call the contact, connecting the two to make a call. First, I would suggest adding important phone numbers through GrandCentral's web interface. All you do is go to the "Address Book" tab, then click on the "Add" button under "Add a Contact."
Once your contacts are added, you can access this address book through a mobile interface on your phone. I would suggest bookmarking "http://grandcentral.com/mobile/contacts" in your phone's browser bookmarks. Once you sign in on the mobile site and go to the "Contact" section, you can click on a contact and then click on the "Call" link. This duplicates the same function as the full site's call button, and GrandCentral will call your SIP number, connecting the call.
Because Nokia allows your Gizmo number to be integrated into the device as if it were any normal number, the call from GrandCentral will come in like a regular call. The only difference is that your GrandCentral number will be followed by an "@ip.address." If you configure your GrandCentral account to display the caller ID as itself, and not the number calling in, you can add GrandCentral to your default phone address book and have it appear when a call is made. Note that the icon in the top right of the status bar (with a phone and a globe) is the icon that indicates the SIP service is running.
Though it may not be streamlined and refined, this system works great for free calls from anywhere in the world. Though GrandCentral has hinted that it may charge for the call-out feature once it goes out of beta, the service is absolutely free for the time being. Users have also made a GrandCentral dialer application for Nokia Internet Tablets. Though the tablets run a variant of Linux, this application appears to be built on Python. If someone ported this program (on Python) to S60 devices, then Nokia Smartphones may become the best internet telephone devices on the market.
If you have any questions, or need any clarification, feel free to ask your questions in the comments below.