How to estimate Concurrent Calls (CC) in MagnusBilling and simulate CPS × ASR × ACD

When someone asks “how many concurrent calls can my server handle?”, the answer depends on a combination of three variables: CPS, ASR, and ACD. In this article, you’ll learn what each one means, how to calculate CC (concurrent calls), and use an interactive simulator to test real scenarios.

Quick definitions

CPS (Calls Per Second): how many call attempts per second your system can originate.
ASR (Answer Seizure Ratio): the percentage of attempts that become answered calls.
ACD (Average Call Duration): average duration of answered calls (in seconds).
CC (Concurrent Calls): how many connected calls are active at the same time.

Key point: the only variable that directly depends on your server (e.g., MagnusBilling) is CPS. ASR and ACD depend on carrier/route quality and user behavior.

Formula (Little’s Law applied to telephony)

CC = CPS × (ASR/100) × ACD

Interpretation: CPS × ASR gives the number of answered calls per second. Multiplying by ACD (in seconds) yields the average number of connected calls simultaneously (CC).

Practical examples

Scenario	CPS	ASR	ACD (s)	CC = CPS × ASR × ACD
Robocall (weak route)	40	10%	15	60 (40 × 0.10 × 15)
Human agents (good route, long calls)	40	40%	60	960 (40 × 0.40 × 60)
Reported test likely with measurement error	40	70%	6	168 (expected). If someone reported “24”, review the data collection.

How to increase CPS (server capacity)

To raise CPS, scale your architecture with a SIP Proxy Master + Slaves. The Master distributes attempts across Slaves, increasing total origination rate without overloading a single node.

Read the guide: Slave Server (high performance / SIP Proxy Master & Slaves).

Field rules

ASR improves with better routes, valid CLI, anti-fraud, and sane redial policies.
ACD increases when calls are useful (clear IVRs, low latency, stable audio).
CPS depends on MagnusBilling sizing, database, network, and SIP Proxy architecture.

FAQ

1) Can I “fix” a single CC number for my server?

No. CC changes according to the combination of CPS, ASR, and ACD. The server directly impacts only CPS.

2) What influences ASR the most?

Route quality, valid CLI, anti-fraud, call times, DNC, and reasonable redial policies.

3) How to measure ACD correctly?

Use answered calls only, in seconds. Do not include no-answers in ACD.

4) Need help sizing Master + Slaves?

Get in touch via: SIP Proxy Master & Slaves.