In a conventional telephone system implementation, the computer network is completely separate from the telephone system. One of the key arguments made by IP-based telephone vendors is that this is an unnecessary duplication of cabling, arguably a fair point when considering that the cost of cabling an office is typically in the order of $125 - $200 per point. Many IP-based telephones (whether handsets of an IP telephony system or VoIP telephones) are able to share a single network cable with a computer, effectively halving the cost of cabling an office.
A conventional telephone system implementation provides a time-proven, robust and reliable voice communications facility. Call quality is typically extremely good.
A conventional telephone system implementation relies on the business’ ability to successfully negotiate reasonable rates for calls. Invariably, these rates will be higher than may be possible using VoIP systems.