It could also be tricky to apply special policies to different family members under the same plan, said Joanne Steinberg, director of strategic marketing at mobile carrier management vendor Tekelec. For example, parents might want to restrict the kinds of apps that a child can use on his or her phone. Those policies are per user, not per account.

As a result, carriers offering these plans have to modify their OSS/BSS (operations support system/business support system) software and other systems. Specifically, they need to change the way their subscriber database talks to their policy server and their charging system, Steinberg said. Underneath a multi-user, multi-device plan, each user may still need an entry in the subscriber database. Whatever that user does is governed by individual policies but charged to the shared bill.

The new plan type also changes the way phone numbers relate to accounts, Sharma said. In traditional mobile plans, one phone number is linked to one subscriber and account. With bucket plans, multiple numbers and device identifiers have to be associated with a single account. That work can also be done in the OSS/BSS software, so the switches out on the network don't have to be updated, he said.

Rolling out multi-device plans shouldn't force carriers to rip out their back-end servers, but rather require changes to the software that runs on them, Sharma and Steinberg said. Carriers may use different vendors for each, which adds complexity, according to Steinberg. But there are standards for communication among such back-end systems, she added.

Reworking its management system may have taken Verizon some time, but that probably isn't why the new plans are coming out later than many people expected, they said. No other vendor offered them, so Verizon could wait, Sharma said. He thinks Verizon was first out with multi-device plans because it wants to catch up with AT&T on so-called "connected devices," most importantly the Apple iPad, which were available with AT&T radios about a year before they came out for Verizon with the iPad 2, he said.