What about hardware based IP PBX
While the hardware strategies from proprietary vendors haven't been entirely fruitless, they haven't exactly reached the level of functionality for IP telephony that software has. Nor will they. Hardware simply doesn't offer the IP flexibility that open standards software does, and in fact often requires bolting on additional servers to fit a proprietary IP PBX into a voice over IP network. So it's somewhat surprising that the proprietary folks claim their IP offerings are just as functional and easy to implement as the ones from the innovators offering software and standards-based solutions. You should therefore look under the hood to determine exactly what’s behind that dial-tone.

Migration
In an ideal technology world, implementing an IP infrastructure works best if it's a straight-up installation enterprise-wide. Let's say that's the right way — yet not always realistic since most organizations that already have TDM-based PBX phone systems can't afford to just take them out of commission and start from scratch: Cost-wise, or downtime-wise.

For companies who have existing investment in private branch exchange (PBX), it may be not be advisable to switch to IP PBX overnight. Instead, UBM recommends the strategy of migrating to IP telephony over time.
The best approach to migrating to IP telephony over time, then, is to develop a migration plan that blends existing traditional TDM resources and IP to get started. Scheduling your migration in phases for various departments, workgroups, etc. also lets IT teams work out issues on a small scale and set a better stage or extending voice over IP across an entire organization.
In order to marry a new IP PBX and existing TDM systems, especially in multiple locations, it's important to choose a vendor whose IP offering can be installed on traditional telephony interfaces and migrated to VoIP later without forklift upgrades, no matter the location.