With the T-Mobile Even More Plus 500 plan, you get an additional 500 minutes over and above your 500 contract minutes every month for Call Forwarding. You effectively have 1000 minutes on your plan, T-Mobile just doesn’t advertise it that way. In fact T-Mobile representatives have been instructed to let people know they could just turn on call forwarding and pick up their calls on another phone if they’re about to run out of minutes on their plan for the month, that way T-Mobile doesn’t have to dole out as many bonus minutes.
Fortunately, it turns out that with Voice 0.3.0 as available on the Nexus One (which does some more funky stuff with your phone and network settings than previous versions, e.g. dialing the hashcode to replace your voicemail number at the network level), some of the time or most of the time if you use your Google Voice number as your primary number, the Voice app actually sets up your phone to use Call Forwarding minutes with T-Mobile — and you don’t actually end up using contract minutes at all! I discovered this because I was about to go over my minutes based on calls I made before buying a Nexus One, and asked T-Mobile for bonus minutes, but later found that the minutes I used from that time and after upgrading to a Nexus One on actually came off my Call Forwarding allocation that I didn’t even know I had.
With Voice 0.3.0 on Android-2.1, you can add a shortcut to the home screen that toggles between the three “Use Google Voice for all calls?” settings (Use for all calls, use for no calls, use for International calls only). This gives you quick access to either your normal calling minutes (the 500 on your contract) or your Google Voice minutes (your extra 500 forwarding minutes), assuming you’re OK with the caller ID changing between calls. Thus you get 1000 minutes a month if you’re OK with working with the subtle caveats of switching back and forth.
I say “most of the time” Google Voice uses your forwarding minutes, because I have seen it use non-forwarding minutes on my bill, and I don’t know what caused the difference. But anyway if you’re running out of minutes one month you can in theory just switch over to your original cellphone number and voila, you get 500 more minutes. If you’re over on your Google Voice / Forwarding minutes and people call you, you might have to not pick up the phone and call the person back on your cell number so you don’t go over. Be aware that T-Mobile takes minutes off your plan for both leaving messages on voicemail *and* checking voicemail as far as I know (scoundrels), so your GV number should be your primary number, that way if people call your GV number and you’re out of forwarding minutes, they’ll leave you a message but it (hopefully!!) won’t count as additional forwarding minutes, because Google handles the Voicemail call and it doesn’t even go through the T-Mobile network once you decline the call (I think / I hope. Though the fact that they replace the voicemail provider at the network level by dialing the hashcode might actually mean this is not true. Anybody know?).
Note that bonus minutes do not apply to Call Forwarding minutes, so if you go over your 500 minutes with Google Voice, there’s nothing T-Mobile can do (will do?) to help you. This is true whether or not you use the hack described here to double your minutes, as long as you are using your GV number as your primary number. I suspect this is going to cause a lot of people a lot of problems really soon, because T-Mobile frequently gives out bonus minutes but they can’t/won’t if you have gone over using Google Voice, meaning some people will get slapped with really nasty bills for going over while using Google Voice.
Anyway with free T-Mobile to T-Mobile minutes, free nights and weekends, 500 contract minutes and 500 Call Forwarding minutes, I think that Even More Plus 500 is actually going to work for me now! That certainly was not the case before I discovered this, I typically need to use a lot more on-peak minutes in a month than 500.
NOTE: You should watch your bill closely to make sure that this actually works and is all correct, both in general and for your account specifically.