When it comes to converging fixed and mobile networks that yield seamless user experiences, some enterprises want CPE-based solutions, fearing that waiting for carriers to deliver unified services will take an eternity. Others put their faith in the carriers, because they consider fixed-mobile convergence (FMC) too tough to figure out alone.

In these early days before widespread dual-mode products and services, many companies seem momentarily placated by single-number reachability. Last week, Verizon Business joined the likes of Ascendent Systems, Cisco, DiVitas and FirstHand, as well as direct competitor AT&T/Cingular Wireless, in offering a way to extend corporate PBX calls and dial plans to mobile phones to achieve this capability.

The carrier teamed with its Verizon Wireless sibling to offer PBX Mobile Extension, a service to launch next month. It works with a company’s existing PBX to provide a single direct-inward dial (DID) phone number that simultaneously rings multiple phones, a function often referred to as “twinning.”

To offer the service, Verizon is reselling Ascendent’s Voice Mobility Server, which resides on the customer premises. Mike Marcellin, VP of product marketing for Verizon Business, acknowledged that PBX Mobile Extension doesn’t yet constitute a full-blown managed service, because enterprise IT departments manage the server software themselves.

So what’s the difference between using Verizon’s service and just buying mobility server software from Ascendent (now owned by Research In Motion of BlackBerry fame)?

One-stop shopping for multiple wired and wireless services, says Marcellin.

In addition, like AT&T’s existing OfficeReach service, Verizon PBX Mobile Extension offers calling plans that lower the cost of cell calls made from within certain corporate zones, adds Jim Elter, associate director, wireless business product development at Verizon.

The Verizon and AT&T offerings indicate that the carrier community is coming around to acknowledge that, while there are a number of ways to skin an FMC cat, large enterprises are serious about making PBX DID numbers their employees’ primary business phone numbers.

For smaller businesses, though, Verizon launched a Wireless Office service, which is basically a mobile Centrex service. In this case, Verizon Wireless phone numbers become users’ extension numbers, and Verizon said it will install in-building repeaters to ensure strong in-house signals.

Joanie Wexler is an independent networking technology writer/editor in Silicon Valley.