First I suggest reading the FAQ listed in the first link, as it explains a lot.

EVDO has this ability already built in, so let me explain what it does and currently what 1x does, and you will see the difference.

EVDO if you receive a call while connected and moving data, it will put the data dormant, and let you receive the call.

1x if you receive a call while connected and moving data, it will send the call to voicemail, and won't ring in or do anything.

So if you have this on your account, it will automatically cause the 1x network to go dormant, and you will receive the call. So this will benefit you if you live in a 1x network and you want push e-mail. Without this, if you receive a call while you were downloading e-mail, then it would go to voicemail.

So with this, 1x now works very much like EVDO, except for speed, so you get the benefits of EVDO (again, except for the speed), and this gets rid of the flaws of the 1x network...

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-Michael
Need help setting up your i730/i830 or other WM2003/WM5 Phone, as well as the other hacks I have done, check out the MRailing Guides and Links