What is a PRL? What does it do? How do I update it?
Special thanks and credit to HoFo user meatchicken, whose post here I adapted for this post.
To update you PRL (Preferred Roaming List), simply dial *228 while NOT roaming (it shouldn't matter if you're in your home market or not). It takes about a minute, is a free call, and your phone will reset (turn itself off, then back on) after programming is complete. An automated voice will tell you if the PRL was successfully loaded or not.
If you are in the Chicago area, you will have an odd-numbered PRL. Everywhere else uses an even-numbered PRL. The primary difference is that in Chicago, Verizon and uscc don't roam off of each other. This is not the case elsewhere, as Verizon is our primary roaming partner in many markets.
EV-DO PRL's will begin with 15XXX, 1x PRL's begin with 39XXX. The PRL update frequency varys from a few days to a couple of months, so I won't post the latest version here. Someone will usually post whenever a new PRL is released.
Note: Although EV-DO does have a different PRL than 1x, at this point there is no EV-DO roaming, so that merely enables your phone to see the EV-DO network. When in a non-EV-DO area, or while roaming, you will fall back to the standard 1x PRL.
What It DOES Do:
It loads into the handset, over-the-air, a software "list" of Cellular Systems ("carriers"), that your phone is allowed to scan for, *after* it loses or can't find a uscc signal. Your phone may or may not display "roam" or "roaming", but it will display a triangle (point up) near the signal strength meter. (note: some manufacturers, notably Audiovox, use an "R" instead.)
The PRL list is set up in area groups, & in a priority order, so for example, in City "A", the phone, after losing a uscc signal, will know to first attempt to find Alltel before Sprint, ect.
If uscc has a system in the area, it is always first in priority over other companies such as Sprint or Alltel, and will be looked for first, regardless of weather or not another carrier has a tower closer to your location or a higher signal strength.
Most phones will also re-power up/initialize after downloading the PRL.
USCC seems to release a new/updated PRL every 30-75 Days or so.
Doing a PRL update more often, such as once a week, is harmless & will result in simply writing the same PRL file over itself.
New updates can and have both added & removed "partner" roaming systems, depending on uscc and how they want their phones to roam in any given area.
It may help with border roaming (picking up another carriers tower outside your coverage area).
What It DOES NOT Do:
Contrary to the seemingly endless loads of mis-information & rumor on the net, *228 DOES NOT:
Have anything to do with any individual towers, or accessing newly built towers. If a company builds a new tower near your home, that new tower is already part of an existing system (SID - System IDentification), which either is already listed in the PRL, or isn't. Only entire SID's are in a PRL. There is no way for uscc to add or block any individual new or old towers via the PRL. That's not how the PRL works. Period.
The PRL update can't increase a weak native uscc signal strength issue. If your phone is already locked onto a "1 bar" uscc signal in a given area, nothing in the PRL list is going to instruct the phone to drop its native uscc coverage, & look for another company's signal instead. There are no roaming partners listed in the PRL that have a higher priority than the local uscc signal. However, there are going to be areas within your local coverage area that you will roam occasionally. In this event, the PRL will step in to determine who you roam on, and the billing system will not bill you roaming charges. Also, on a related note, n/w's and incoming will still count while roaming, as long as you are picking up a tower within your local coverage. Other programming in the phone determines at what signal strength the phone will "give up" on native uscc coverage and search for a roaming partner.
The phone's internal PRL list has absolutley NOTHING to do with anything, once you are "in a call", and at that time the PRL is as dormant as a game or other not-in-use software entry in the phone. The PRL is not used at all during an actual call in progress. The cellular system, during a call in progress, has full control, and the system end decides what cells to hand off your call to, and any issues/problems, such as a dropped call, or any other "in call" issues/problems in a given system, have nothing to do with the internal PRL list, & can't be fixed by *228.
Updating the PRL has nothing to due with any problems relating to earpiece volume, battery charging, screen contrast, voice quality ect...except that after a *228 the phone might restart, so any odd quirk that might have gone away by shutting the phone off & turning it back on, may also "coincidentally" go away after a PRL update.
The actual firmware version of the phone can not be changed or updated thru a *228 call, either. Firmware can only be updated in a uscc store or by the manufacturer of the phone.
*228 also has nothing to do with whether or not you are billed roaming. That is completely handled by the billing system, and is completely depending on the "coverage area" associated with the price plan in effect when the call is reported to the billing system. In my experience, that is usually no more than 3-5 days, but, by FCC regulations, can take up to 60 days. Depending on when the call was placed in relation to your billing cycle, this means that a roaming call could take up to 3 bills before it shows up. As I said, it usually won't, but it's good to be aware that it can happen.
Additional technical info, courtesy of our very own towermaster:
Originally Posted by CDG Reference Document #130