How to Choose the Right IP PBX Capacity Based on Extension Count

One of the most common mistakes when buying an IP PBX is picking the wrong capacity, either too small so it fills up fast, or too large so you're overpaying upfront. Here's how to work out what your business actually needs.

Step 1: Count your current extensions

An extension is any internal phone number you need, usually one per employee or one per desk that needs a phone. Don't forget to include extensions for meeting rooms, reception, and any dedicated lines like fax or alarm systems tied into the phone network.

Step 2: Estimate simultaneous calls

The number of extensions is different from the number of calls happening at the same time. An office with 100 extensions won't necessarily have everyone on a call simultaneously, usually just a portion are active at once. This is what determines the "simultaneous calls" capacity you actually need to plan around, not just the extension count.

Step 3: Factor in growth plans

Pick a capacity with a bit of room to grow over the next one to two years so you're not swapping out the system too soon. But don't overbuy way beyond your actual needs either, that's just wasted investment upfront.

Zycoo CooVox capacity guide

• CooVox T100: up to 100 extensions, 50 simultaneous calls, 50 conference participants, 8GB of call recording storage (roughly 400 hours). A good fit for a small to mid-size office.
• CooVox T200: up to 200 extensions, 80 simultaneous calls, 80 conference participants, 1TB of call recording storage (roughly 15,000 hours). Suited for mid-size offices that need more call documentation.
• CooVox T600: up to 600 extensions, 150 simultaneous calls, 150 conference participants, 1TB of call recording storage. A good match for a larger company or head office connecting many branches.

The features stay the same across every tier

All three models support modular telephony interfaces (FXO, FXS, and GSM on the T200 and T600), SIP and IAX2 protocols, USB storage expansion, TLS and SRTP security, and built-in features like IVR, voicemail, call recording, and paging. The real difference is capacity, not feature completeness.