The iPhone 3G automatically switches between 3G and 2G networks for voice and data connections. In theory, the phone should hop off 3G when signal strength is too low or non-existent, and hop back on when the signal gets better. In our experience, however, the iPhone 3G isn't aggressive enough regarding the switch from 3G to 2G. In other words, you may find that your phone stays connected to a 3G network when the signal strength is too low to allow incoming or outgoing calls.
The simple solution is to manually turn 3G connectivity, forcing a switch to 2G -- which may take several seconds -- and likely a boost in signal strength.
A number of other signal-strength-boosting techniques are available, including:
Reset iPhone In some cases, simply resetting the iPhone can resolve signal strength issues. Hold down the home and sleep buttons simultaneously until you see the white Apple logo, indicating that your iPhone has restarted.
Reset network settings Tap Settings, tap General, tap Reset, tap Reset Network Settings. This will cause your iPhone to restart, and will delete any stored Wi-Fi passwords as well as DNS settings and more. It can resolve signal strength issues in some cases.
Reseat your SIM card If your iPhone's SIM card is not seated properly, signal strength can suffer. Try re-seating the SIM by pushing a straightened paper clip into the small hole on the top of the device to open the SIM tray, making sure the SIM Card is properly in place, then re-inserting the SIM tray. Also check for debris inside the tray or SIM card slot.
Replace your SIM card If you've exhausted conventional solutions, a replacement SIM card from your local provider may prove beneficial to signal strength.
Attach tape to your SIM card Rooted in the notion that improper SIM contact can result in a weak signal, reports indicate that attaching a small piece of scotch tape to the outer side of the iPhone's SIM card (the side that does not have metal contacts) can result in a surprising boost. We certainly can't recommend this procedure, as it might void your iPhone?s warranty. But if you've already cleaned debris and made sure your SIM card is properly seated, a piece of scotch tape may very well provide the added pressure needed for proper contact and a strong signal.
Restore the iPhone Connect your iPhone to your system then click the Restore button under the Summary tab in iTunes. Restoring the phone will erase contacts, calendars, photos and other data on the phone (including any third-party applications), but will restore automatically backed-up information including text messages, notes, call history, contact favorites, sound settings, widget settings, etc.
Some users have reported that restoring the iPhone, but not restoring custom settings data from the computer-stored backup alleviates this issue. Note that you'll lose text messages, notes, call history, contact favorites, sound settings, widget settings, etc with this method, though you can restore them anytime by simply doing another restore and choosing to push the backup to the phone.
Dock the phone or just attach a (dangling) USB cable We previously reported that docking the iPhone or attaching it to a host computer via a USB cable (in turn delivering a charge) can boost signal strength dramatically. It's difficult to discern whether the signal is boosted by simply having a cable attached, or whether the power delivered during a charge boosts signal strength.
At least one iPhone Atlas reader found that simply attaching a dangling (not connected to anything) USB cable to his iPhone provided an instant boost in signal strength.
Give the phone a full charge (battery related?) Some evidence indicates that poor signals are a symptom of low battery charges, though this fix may be conflated with the aforementioned: docking the phone or attaching a USB cable.
Exchange for new unit Some users have had success obtaining replacement iPhones for signal strength issues. Though this solution is far from universal, the following case is evidentiary:
Buy a signal booster A few readers have reported success with third-party wireless signal boosters like the $250 zBoost.
Firmware update? It's possible that a future firmware update from Apple will hold the keys to signal strength issues. Stay tuned...