Some devices don't do well on the default slot cycle setting.
That is the setting that determines how often the phone contacts the nearest tower, in seconds, to query for incoming calls; the stock setting is "2," which translates to three times as long as a setting of "0" and twice as long as "1." Those are not seconds, just settings. The actual seconds are longer than "0" for a setting of "0," obviously.
The factory setting of "2" is designed to sacrifice quick caller to receiver functionality a bit in order to save battery life. The lower settings keep the device busier than the higher settings, so prudent battery saving measures taken in other areas of the system should be taken, such as lowering the screen brightness, leaving GPS off until actually needed, etc (see
http://androidforums.com/htc-droid-eris/19176-optimize-your-droid-eris-battery-life-read.html).
To adjust the slot cycle, you must access the slot cycle index, which is accomplished by doing the following (be patient between steps, a bit of a delay occurs sometimes):
- press the "phone" button
- dial ##778
- press the green "call" button
- choose "edit mode"
- enter six zeroes as the password (000000)
- press modem settings
- press slot cycle index
- make your selection
- press "menu"
- confirm your setting
The device will reboot.