Check It Is Really An Open Standard!
Many system integrators and manufacturers will ship you 'standards based telephony systems' with additional 'configuration and management tools'. Carefully analyze whether or not these standards and tools are fully conformant with the IEEE or RFC standards. For example, SIP is sold by Cisco and Avaya. Not both brands are compatible with open standards, and both vendors have different licensing models. Also remember that if you choose an open source solution like the Asterisk PBX, you may find that when a solution provider gives you configuration, reporting or management tools, they may well actually be providing you with a closed source, proprietary solution which will mean that you are locked into their support and development system (and associated costs) rather than being able to have another Asterisk technician work on your system. Sometimes you will need to use a closed standard, but before you do, make sure it is really necessary. There are many support specialists who can provide most of what you need using open source software.
Host On A Separate Network & Server
Ok, this is highly controversial. But, I don't think anyone would argue that if you want to make 100% sure there aren't any problems, you need to have a separate voice network (so voice runs on one physical data network and data on another physical network) and a separate server to run your VoIP software. There are skilled companies who can get both data and voice to run on the same network properly, but these people are hard to find. These skilled companies use intelligent switches (not the sort you have at home) and will create VLAN's (virtual local area networks) as well as using other protocols like QoS (quality of service) to create a logical / virtual separation of voice and data, yet it will run on the same physical network.
Many people lose sight of the fact that your PBX runs all the communications for your business. You have had a dedicated box on the wall for many years now to do one thing: communicate. Why should it be any different now that you have the option to run your communications on a server? Also don't forget, you may start out with only one or two things on the same machine (for example web and VoIP) but you may find that you may start setting up other processes (such as database and email) on the same server as your voice and you will start to notice the quality degrade when resourcing (RAM, hard disk drive space or usage, processor usage) becomes stretched.
Choose Good Hardware
Because people can now choose the underlying hardware behind their phone system, there can be the temptation to 'skimp' on the machines specifications and quality. Resist at all costs! One failure can cost your company thousands, possibly millions. (Ok, it would be thousands for a small company and potentially millions if you are a company the size of an international bank.) To ensure your communications work day in, day out and work reliably, use the following guide when planning to purchase a VoIP solution from a vendor (includign those that provide their own 'server'):
- Ensure the hardware is from a brand name vendor, not a white box from the store around the corner.
- At a minimum, ensure the server has a RAID, and if possible has a backup 'hot swap' power supply to ensure that your server keeps running and providing communications to your organisation.
- Ensure that you have an un-interrupted power supply (UPS) which is capable of running the server, any network equipment required for your telephony, and your phones for at least 30 minutes to 1 hour, more if you don't have an onsite telecommunications / data technician.
- Ensure that your network switches (and your phones) are PoE (Power over Ethernet) so that you can connect the switch to an un-interrupted power supply (UPS) and your phones will work when the power is off.
Flexibility & Support for Products
Your final VoIP solution should support multiple vendors equipment (for example different brands of IP phone) and should allow you to choose which service provider will support your system. Within the field of Asterisk (one open source PBX platform) you can find literally thousands of consultants who can help program the system to achieve your desired outcome. You can also put someone on staff in your IT department who can help maintain the system.