The mobile network operators have been quietly enhancing their cellular voice plans to make using their wireless networks for intra-company calls more economically appealing. For a couple of years, the big U.S. operators have offered calling plans that integrate, to a degree, your cellular phone with your PBX feature set and on-net campus calling price plan. Now, those options are becoming more numerous and varied.

Most recently, Sprint Nextel added Sprint Wireless Integration with Cisco Unified Communications Manager to its portfolio. The service, announced in November, requires that you use Sprint’s Multiprotocol Label Switching (MPLS) wireline service. It counts calls made from a corporate mobile phone into the company and vice-versa as “on-net,” according to Sprint’s manager of fixed mobile convergence, Dan Jacobson.

Sprint Wireless Integration for use with Avaya IP PBXs is also available.

The service is a feature you add to an existing Sprint wireless voice plan, with a per-user/per-month surcharge for the feature and a 100-user minimum. The offering joins Sprint’s existing $8-per-month-per-user Mobile to Office service, which lets employees make mobile-to-PBX calls that don’t count against minutes on your pooled mobile minutes plan.

Meanwhile, AT&T has been ratcheting up its offerings. When its OfficeReach plan was introduced in 2006, it gave per-minute discounts to cellular calls made from physical corporate campuses, a.k.a. “zones.” Since then, AT&T has done two things:

1) Made OfficeReach a “zone-less” service. In other words, the service doesn’t discriminate between on-campus and off-campus calls, so long as you’re between corporate numbers across the AT&T Mobility network, explains Lourdes Charles, OfficeReach product manager.

2) Announced AT&T Mobile Extension in September for providing PBX-to-mobile calling discounts. This service costs about $15 per user per month for 500 minutes of talk time, or about 3 cents per minute. You must have a Cisco IP PBX for this to work, though support for Avaya IP PBXs is coming soon, Charles said.

For its part, Verizon Wireless offers the Verizon Wireless Office service, for which you can elect two components:

1) On-campus pricing (calling to and from your campus locations as defined by triangulation of VZW cell towers).

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