This list tells your card which towers to look for when establishing connection, including towers from other carriers that Sprint/Verizon has roaming agreements with.
As new EVDO towers are installed or new roaming agreements are reached, your PRL will become out of date because it won't know to hunt for these new towers.
A Hands Free Activation encompasses both a Profile and a PRL update, as well as a Firmware check on most phones.
All Verizon and Sprint EVDO modems have a Preferred Roaming List (or PRL) embedded in the card.
The Preferred Roaming List (PRL) is a database residing in a wireless (primarily CDMA) device, such as a cellphone, that contains information used during the system selection and acquisition process.
In the case of R-UIM-based CDMA devices, the PRL resides on the R-UIM.
A PRL update provides your phone with the most up-to-date info regarding roaming and the roaming network.
In the US, for Verizon / Metro PCS / US Cellular, this feature code is *228 (*ACT).
You may be in an EVDO area, but if your card doesn't have the tower in the list, it won't know to attempt to connect to the new tower!
Your EVDO modem does automatically "know" to add new towers to its PRL, so it is highly recommended that you manually update your PRL monthly so that you can ensure that your EVDO device is always able to connect to the currently available EVDO towers.
On many networks, regularly updating the PRL is advised if the subscriber uses the device outside the home area frequently, particularly if they do so in multiple different areas.
This allows the phone to choose the best roaming carriers, particularly "roaming partners" with whom the home carrier has a cost-saving roaming agreement, rather than using non-affiliated carriers.