The Preferred Roaming List (PRL) is a list, in sections by geographic area, of network numbers that the phone is allowed to connect to. The PRL on the phone is provided by the original mobile operator, Verizon, and lists Verizon's networks but not their hated rivals.
My original goal was to avoid paying for services I don't use, by using a prepaid plan with a modest but sufficient data allowance. Boost Mobile appeared to be the optimal mobile virtual operator for this purpose; it resells Sprint service and in fact is a subsidiary of Sprint.
This phone is unlocked for using GSM, and forum posters in Europe report that GSM operation with a variety of GSM mobile operators succeeds. As a test, while my AT&T account was valid, I inserted my AT&T SIM in the phone and tried to use the network, but without success. I understood that a GSM phone should obey the PRL on the SIM. However, forum posters suggest that the phone coprocessor software on the Droid 3 (and Verizon phones in general) checks for rival USA mobile operators and specially suppresses them. Alternatively, the PRL can include negative entries, and it is possible that the rival networks are suppressed in this way.
To use this phone on Sprint, I am going to need to replace the PRL. Here are notes on this topic:
http://www.roamingzone.com/prl/ -- These guys post interpretatations of recent PRLs of major carriers. As far as I can see, Verizon does not include Sprint, AT&T or wildcards in their PRLs. There are several PRLs; which one you get depends on your plan.
To get an updated Verizon PRL onto a Verizon phone, dial *228 and selection option 2. (Option 1 is for activating the phone, and it does other steps in addition to downloading the PRL.) Other mobile operators have their analogous numbers; for example, Sprint's is ##UPDATE# or ##873283#. Of course the objective is to install another operator's PRL.
PRL Update on HTC Touch -- By jj62mtp probably dated 2009-11-11 and updated 2011-06-16. Has attached a (recent?) Sprint PRL, and instructions how to update it on the HTC Touch (Microsoft Pocket PC?) But this is assuming that the HTC Touch is already activated on Sprint.
Flashable PRL --
#Winning# 2011-03-07 posts a flashable zip file with a (sprint?) PRL in it.
Also people in this thread suggest reverting to a stock image and forcing
it to update the PRL. (But that would get me a Verizon PRL.)
The zip file just contains the PRL. It's all binary stuff. It's not clear
whether Recovery knows the special procedure to pass the PRL to the phone
coprocessor, referred to as
Flashing NV Item in a OTA update.
PRL on HTC Hero -- kyouko 2011-07-09 has a guide for forcing a PRL onto the HTC Hero (with Qualcomm processor). This involves running Qualcomm's QPST diagnostic software on Windows XP. You need the Service Programming Code, also known as the MSL, and a link is given to an app that will figure out the MSL. This person switched repeatedly between different networks. But the Droid 3 has a TI OMAP 4430 system-on-chip which includes a separate core as the phone coprocessor. Not likely that QPST would know how to work with it.
It looks like installing a rival's PRL is not going to be as easy as I thought. And there is another fly in the ointment: Boost Mobile refuses to activate the phone, realizing from the MEID that it wasn't sold by them. After a considerable amount of thrashing around, I finally decided to cut my losses and to be a loyal Verizon customer, at least temporarily.
As for plans, the best one is the prepaid daily plan, $0.25/minute plus
unlimited data for $1 on each day used. However, Verizon refuses to
sell this one to me; it's for dumb phones only. If you're over 65, look for
Nationwide 65 Plus plans which give you less usage for less cost,
even though still expensive.