• Number of VoIP Users – This can be determined by the number of SIP extensions you will have in your solution. Most open source telephony software applications allow you to add as many VoIP Users or SIP extensions as you wish. No hidden licensing fees or additional costs. The number of users may affect however, the maximum number of simultaneous calls you may have, so please read the next section that discusses this.
  • Maximum number of Simultaneous Calls – This number can differ from the number of VoIP users you have on your system. For instance, you may have 50 VoIP users on your system, but only have a total of 10 concurrent calls at any given time). A good number to go by is 23 concurrent calls. 23 concurrent calls require at least 1GB of RAM. 46 Concurrent calls requires about 2GB or RAM, and anything above that, you will be looking at 4GB of RAM. Keep in mind, it never hurts to have more RAM than not enough. This can directly affect your call quality and processing power of your server.
  • Hard Drive Space – Are you a company that records many if not all call’s or allows VoIP users to archive VM messages a long period of time, if so, you may need a larger Hard Drive space to accompany this data. Most VoIP servers are running 80GB to 160GB SATA HD’s. If you are a company that does a high amount of call recording or VM storage archiving, you may want to consider 500GB to 1TB HD’s. Another option may be to store these recordings or messages on a separate network device such as a NAS that contains these high density HD’s or an FTP server, instead of storing everything on your server HD’s.
  • Redundancy from a data perspective – This requirement should apply to everyone. I don’t know of any customer that doesn’t want a RAID setup for the dual HD’s to essentially back-up system data and software in case one of the HD’s fails. RAID1 allows you to do this seamlessly as data and information is simply mirrored from a primary hard drive to a secondary hard drive for back-up purposes. Like I stated above, RAID1 from a data redundancy perspective is strongly suggested. In most RAID1 applications, you will setup 2 HD’s, a primary and secondary. You will setup a mirror which essentially copies all of the data from the primary HD to the secondary HD. In a situation where a HD fails, you have a full backup on your second HD, and will minimize server down-time.
  • Redundancy from a power requirement perspective – Dual hot swappable power supplies provide a redundant power source if 1 power supply fails. Even better, a replacement hot swappable power supply can be ordered and replaced as easy as sliding the bad power supply out and popping the new in and reconnecting the power source. Let’s face it, power supplies fail. If your server is only running from a single power source, and it fails, you physically need open your server, and replace it or worse yet, replace the whole server. With a redundant power supply option, if one fails, your server is not affected and will continue to remain in production on a single power source, until you can replace the failed unit. In most cases, these are also hot-swappable, and are replaced as easy as “sliding in a new dresser drawer”.
  • PCI requirements – If you are going to connecting to the outside world via a T1 digital or PSTN analog trunk, one of the best solutions to use is a digital or analog telephony PCI card installed in your asterisk server. You will want to make sure that your server provides the necessary PCI or PCI Express bus on the motherboard, and then ensure you purchase the correct telephony card for your application.
  • Space both inside the server chassis and rack space – If your VoIP application requires more than 2 PCI or PCIe telephony cards, it may be beneficial to go with a 3U server as it contains the necessary room inside the chassis to accompany these cards. Also full length PCI or PCIe cards may not physically fit in a smaller chassis such as a desktop and even some 1U servers. 3U servers also provide better ventilation for cooling as telephony cards, especially FXS do dissipate heat. It is suggested to keep your server running as cool as possible and heat could impact the processor, slowing everything down. Rack Space is another requirement to consider. Do you have the room in your server rack for a 3U (3 inches in height), if not maybe you should look at a 1U (1 inch in height) server. Keep in mind however, your PCI requirements when you make this decision. Servers come in many shapes in sizes. The most common telephony servers consist of a 1U, 3U, or desktop mini appliance which is about the size of an Xbox. 1U and 3U chassis are characterized by how much rack mount space the take up. In relation to this, a 1U server is about 1Inch in height and a 3U server is about 3 inches in height. If you have the space, a 3U server is always suggested as most 3U servers have more PCI and/or PCIe slots available on the motherboard as well as physical slots on the chassis itself. This will permit you to use up to 6 analog telephony cards or full length telephony cards without the installation hassle you may have on smaller chassis such as 1U or Mini appliances. 1U servers usually facilitate up to 2 telephony PCI or PCIe cards with use of a PCI riser card and expansion.

Software tested:
Open Source PBX – Asterisk
Consilient Tech (16 Cores Xeon CPU, 20 GB DDR3 -> up to 100 simultaneous calls)

Source: https://goo.gl/YB153H