The last newsletter touched on several emerging alternatives for integrating low-cost Skype IP telephony service with enterprise-class smartphones and PBXs. Beyond Skype, there are other crafty ways to achieve cellular telephony savings, as well.

For example, there are cellular gateways available that connect to your PBX. These gateways basically emulate a cellular phone; they pick up a call that has been routed to the PBX and, from there, transfer the call to a mobile number. In this setup, the call actually appears to be a mobile-to-mobile call. The carrier network sees the call as being between the gateway (which the network sees as a mobile device) and the call recipient’s mobile handset.

Mobile-to-mobile calls usually are free under negotiated corporate cellular pricing plans. In other words, they don’t count against your corporate pool of cellular minutes. So turning a wired PBX–to-mobile call into a mobile-to-mobile call should slow the drainage of your mobile minutes pool.

This setup might be attractive, for example, if you want to keep a toll-free headquarters phone number for your main business that customers can call. But let’s say the 800-number caller needs to be transferred to an employee in the field who is carrying a mobile device. In this case, you’d pay only the toll-free service charges on their behalf, rather than paying both for the toll-free usage and mobile usage.

Cellular gateways, available from companies such as Telular and ITS Telecom, generally start at about $300 and can run into many thousands of dollars, based on the number of cellular ports you wish to support.

In addition to the cellular gateway trick, the wireless carriers have been beefing up their discounts for on-net traffic with variations of their intra-enterprise cellular plans. It pays to stay on top of those and get the most that you can out of them.

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