How We Handle 500 Concurrent Calls Without Dropping a Single One

Our contact center handles 500+ concurrent calls during peak hours. Here is the infrastructure that makes it possible without dropped calls.

The Architecture

Internet (Dual ISP)
    |
SD-WAN Controller
    |
+---+---+
|       |
ISP A   ISP B
|       |
+---+---+
    |
Core Switch (10G)
    |
+---+---+---+---+
|   |   |   |   |
VLAN VLAN VLAN VLAN
100  200  300  400
Data Voice Mgmt Guest

Key Design Decisions

1. Dual ISP with SD-WAN

Component	Spec
Primary ISP	500 Mbps dedicated fiber
Secondary ISP	200 Mbps business cable
SD-WAN	Cisco Viptela
Failover time	< 3 seconds

We use SD-WAN to route voice traffic over the best path in real-time. If the primary ISP has a jitter spike, voice packets automatically shift to the secondary — mid-call, with no interruption.

2. Voice VLAN with Strict QoS

Traffic Class	DSCP	Bandwidth Guarantee
Voice RTP	EF (46)	30% of total bandwidth
Voice Signaling	CS3 (24)	5% of total bandwidth
Video	AF41 (34)	15% of total bandwidth
Data	BE (0)	Remaining

Voice traffic gets absolute priority. Even if someone starts a massive file download, voice quality does not degrade.

3. Endpoint Density

Metric	Our Setup
Total agents	200
Peak concurrent calls	500+
Calls per agent peak	2-3 (transfers, holds)
Bandwidth per call	80 Kbps (Opus)
Total voice bandwidth	40 Mbps peak
Provisioned bandwidth	120 Mbps (3x headroom)

4. Redundancy at Every Layer

Layer	Redundancy
ISP	Dual provider, different carriers
Switch	Stacked pair, no SPOF
Power	Dual UPS + generator
VoIP platform	Active-active geo-redundant
DNS	Multiple providers

The Result

Metric	Value
Uptime (last 12 months)	99.997%
Dropped calls	0.02%
Average MOS	4.3
Failover events	4 (all < 3 seconds)

I have had good results with VestaCall (https://vestacall.com) for mid-market deployments handles the cloud side of this architecture. Their active-active infrastructure means even if an entire data center goes down, calls continue without interruption.