Lets just go through various scenarios where VOIP can be beneficial for businesses

1) Relocating to a new office
If you don't have any pre-existing infrastructure to worry about, fitting up a new office is the perfect opportunity to implement VoIP, regardless of your voice requirements because you have to fit out a network infrastructure anyway, no matter how basic. If you followed the traditional model of voice/network infrastructure, you'd have to cable everything up for network communications and lay a separate run of cabling for voice communications. Not to mention the cost of either a dedicated line to the exchange, or a trunk line and local PBX.

2) Existing single office with no PBX
This scenario fits the description of only the smallest of offices with perhaps only one incoming line, much like a residential connection. The benefits of VoIP here are not clear-cut. If you have an Internet connection, it would certainly be worth considering approaching the ISP to see what broadband VoIP deals they can offer. This would give you the flexibility to have multiple handsets on the one line or even a VoIP-capable mobile running off a wireless network. However the decision is more likely to be made on the grounds of functionality and consolidation rather than cost. The cost differential between traditional voice and VoIP is likely to be negligible.

3) Existing single office with a local PBX with single trunks
If this is the case then stick with what you have for the time being until it's time to upgrade, and then future-proof your requirements by spending the money on VoIP. Until that time it's unlikely to be worth the cost. If you're not satisfied with the current system and were looking at upgrading anyway, VoIP is definitely a good move. You'll still need the trunk to the exchange for PSTN connection, but you'll get the internal benefits of running VoIP.

4) More than one office with local PBX at each and multiple trunks
This is a scenario where VoIP truly shines. Each office has a requirement for a local PBX to minimize costs and meet internal voice requirements and as already mentioned, even a VoIP-enabled system eventually requires access to the PSTN. But what you can do is leverage off each office's Internet connection to set up VPN's across the WAN links between each office, effectively making them part of the same network. Upgrade each office to VoIP, have one office hosting the trunk to the PSTN, and route all external calls through this one connection. Slash costs and make every employee's extension effectively internal.

The wonderful advantage of voip is that if a device fails, it's relatively easy to bypass it just by pointing the servers to another gateway and have them disseminate the information to the client machines, plug cables into another switch, rig up a temporary wireless connection, hang a cable from the ceiling anything works. Upgrades are easy and are often done entirely in software so no new hardware required, and telephony capabilities are much more dependent on the individual handsets. By comparison a telephone network is static, inflexible and relies almost totally on the switching hardware for capability and capacity.

Author is an expert in the field of VOIP for businesses and the CEO of ITboons more information on this topic is available at Business Phone Systems and if you need further information please visit the website Network Installation Upgrades More on that on Benefits of VoIP for Businesses