VICIdial SIP Trunk Capacity Planning

Originally published at vicistack.com . Follow us for more call center engineering content. Last updated: March 2026 | Reading time: ~22 minutes SIP trunk sizing for a call center is not the same as SIP trunk sizing for an office. Office workers spend 10-20% of their time on the phone. Call center agents spend 60-80%. An office with 50 employees might need 15 SIP channels. A call center with 50 agents needs 75-125. Get the sizing wrong and your agents hear busy signals, your predictive dialer starves, and you're paying for emergency channel upgrades at carrier rush rates. Most VICIdial admins size their trunks by gut feel: "we have 50 agents, so maybe 100 channels?" That's not terrible, but it's imprecise. The real answer depends on your dial ratio, answer rate, call duration, and how much blocking you're willing to tolerate. Here's how to calculate it properly, with the same formulas telecom engineers have used since 1917 (seriously — the Erlang formula is over 100 years old and still