One cable systemOne of the benefits of VoIP, a single cable system to install and manage, can also be a weakness. If the network is down both your data and telephone systems are out of action.
IT is not telephonyTelephone systems normally have 5 9's (99.999%) uptime. Data networks can't say the same. To put this in perspective, 5 9's means that the system can be down only 5 minutes per year. IT personnel will need to take extra telephony training and be cautious around their networks. No more playing around with the server in the middle of the day. Data networks are also notorious for weak security and viruses. These will impact negatively on the telephone service.
Upgrade the systemBesides the telephone equipment itself, the network infrastructure will probably need to be upgraded. This may include upgrading the network cabling and devices such as hubs and switches to handle fast Ethernet. Routers will need to be upgraded to handle quality of service.
Power can't be interruptedAnalog telephone sets receive power over the telephone wire. This provides dial tone even when there is a power outage. In order to provide this same service, servers and other network devices need uninterruptible power supplies (UPS). IP telephone sets will need to rely on the building's protection from power outages or embrace Power over Ethernet.
911Emergency services depend on being able to find someone who dials 911. This is not a problem with the PSTN since telephone numbers, including extension numbers in office buildings, are mapped to physical locations. IP telephones can easily be moved from location to location. When plugged in, they receive their IP configuration from a DHCP (Dynamic Host Configuration Protocol) server and they are back in service again. Some method must be found to map IP addresses to physical locations in order for Emergency services to respond quickly.