While the industry pushes IP as the future of telephony services, network professionals who manage business phone networks say hybrid IP\/legacy PBXs are helping introduce productivity gains and cost savings without forcing networks to undergo dreaded R&R - as in "rip and replace" - upgrades.Most major PBX vendors have long offered IP options on their legacy gear. Early on, this let users tie together PBXs via IP over private WANs - converged voice\/data T-1s and free inter-company long distance were among key drivers. This hybrid approach, mixing IP and legacy TDM technology, is now extending into the areas of employee productivity and new applications in some companies.Hybrid IP\/TDM voice switches are typically legacy PBXs with digital phones and ISDN interfaces that are IP-enabled - with cards that put the PBX on the LAN (similar to a server network interface card) or gateways that translate voice signals between IP and TDM.The IP-enablement approach lets users keep TDM handsets on desktops, while giving them computer telephony features - such as click-to-dial from a PC and unified voice\/e-mail. IP phones can connect to the PBX via gateways and installing software on the PBX that lets it recognize IP endpoints as digital extensions.With true hybrid PBX gear, the phone switches can handle TDM-based or IP-based handsets, connecting to IP phones via a LAN interface and digital phones through a regular telecom rack. These devices see both kinds of handsets as equals on the network - limitations of each technology notwithstanding.Third-quarter 2005 market estimates from Merrill Lynch show IP telephony systems growing at 31% from the same quarter a year ago, while TDM PBX sales dropped by 20%.Avaya is one company following the hybrid telephony trend in the industry - moving towards IP while maintaining TDM presence. Merrill Lynch says Avaya's TDM PBX sales shrunk 3% from the second to the third quarter of 2005, and compared with the same quarter a year ago sales are down 20%. Meanwhile, its hybrid IP voice sales grew 14%.But analysts say Avaya's TDM business brought in an average of $100 million to $150 million per quarter over the past two years. No vendor of telephony gear would sneeze at that revenue, however much its marketing material looks like an all-VoIP manifesto.A hybrid Avaya PBX at Quaker Chemical, a chemical manufacturing company in Conshohocken, Pa., gives employees the ability to work from home and inexpensively have in-office extensions follow them home.Some users install islands of pure-IP PBXs among a larger TDM infrastructure, usually in branch sites or small remote offices. Such proof-of-concept rollouts are common, but others find integrating IP into the larger PBX network is more effective."It's a useful tool," says Irving Tyler, CIO for Quaker Chemical. Some employees use all-IP in their homes, with an Avaya softphone to VPN-connect into the main PBX. Others use softphone software only for call control, where call setups can be made from anywhere but the voice links are terminated by the PBX, with voice running over TDM internally and the public switched telephone network externally. Such features would be more expensive and complex to set up in a pure-TDM environment, Tyler says.However, swapping all IP phones with Avaya digital sets is not in Tyler's immediate plans. While features such as click-to-call and find-me-follow-me are nice, they're not essential to his business."That kind of flexibility is pretty neat but how many people really need" most of it, Tyler says. "I struggle to see that IP will be completely domain. There are unique features of IP that regular phone can't give you, but those probably only apply in unique situation."At the Southern Company, a conglomerate that manages energy utilities in five Southern states, hybrid IP\/TDM telephony is part of a project to enhance an existing Siemens TDM phone network to bring new applications to the company's voice mail system. (Click\u00a0here to see a diagram of The Southern Company's hybrid unified messaging network.)The project mixes the older Siemens\/ROLM voice mail system with an Asterisk open source IP PBX system, which runs on a Linux server."It's breathing new life into our legacy messaging system," says Arnold Solomon, IT architect for the Southern Company.Under Solomon's setup, a user leaving a voice mail for another employee has the option to send an e-mail page alerting the recipient to the message. Selecting this option triggers the Asterisk server, which talks to the Siemens\/ROLM system via an ISDN interface card on the box. The Asterisk server, running an SMTP daemon, sends an e-mail to users' BlackBerries, cell phones or desktop e-mail clients, alerting them to the voice mail. Solomon also plans to use the hybrid Asterisk\/Siemens setup for group text paging of users' BlackBerries, in which one employee can send numeric pages to teams of BlackBerry users via a Siemens desktop phone.In the past, installing this type of computer\/telephony server integration required consultants and custom programming. "That's where Linux comes in," Solomon says. "This is pretty easy stuff to do," once standard, IP-based hardware and software are thrown into the TDM mix, he adds.Hybrid optionsWhile large PBX vendors tout their pure IP PBXs, many VoIP beginners are getting their feet wet with hybrid systems that switch voice over TDM and IP. Here are some of the hybrid PBX options from telephony vendors.VendorProductProduct descriptionAvayaIP-enabled Definitys8700TDM-based PBX, can support IP phones via an IP gateway or interface card.Linux-based IP PBX, can support digital and analog phones natively.ALCATELOmniPCX4400Hybrid PBX with a native TDM bus and built-in IP switching interfaces for both IP and digital phone support.NortelIP-enabled MeridianCommunication Server 1000PBX supports IP phones via VoIP gateways and IP trunks.IP PBX that can support Nortel legacy phones through a digital interface card.NECNEAXHybrid IP PBX that supports IP and\/or digital phones via interface cards.SiemensHiPath 4000Unix-based hybrid IP\/TDM product, supports VoIP and digital voice natively.