VOS3000 SIP Call Flow – Complete Routing Process with Error Troubleshooting
Understanding VOS3000 SIP call flow is essential for troubleshooting VoIP issues. Every call that passes through VOS3000 follows a specific path from the originating device through the softswitch to the terminating gateway. This guide explains the complete call routing process, identifies common failure points, and provides troubleshooting solutions based on official VOS3000 2.1.9.07 documentation.
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Table of Contents
🔄 VOS3000 SIP Call Flow Overview
In VOS3000, call routing is the process of matching an incoming call to a routing rule that defines which outbound gateway should be used. The softswitch acts as the central intelligence, processing SIP signaling, applying business rules, managing billing, and connecting parties. Here’s the complete flow:
Validation: VOS3000 validates against account database
200 OK: Registration confirmed, client is now “Online”
If registration fails, check: correct credentials, account status (not locked/disabled), IP address matches gateway configuration, and network connectivity.
Step 2: Call Initiation (SIP INVITE)
When the caller dials a number:
INVITE Request: SIP client sends INVITE with called number to VOS3000
SDP Contains: Codec preferences, RTP port for media
VOS3000 Processing: Identifies calling account from source IP or authentication
Step 3: Prefix Matching & Routing Decision
VOS3000 applies routing logic to determine the destination:
Number Analysis: Extracts prefix from called number
Prefix Match: Matches against routing gateway prefix configurations
Gateway Selection: According to VOS3000 manual, gateways are chosen based on: priority number, ratio of current calls to channels, historical calls, and gateway ID
LCR Application: If enabled, Least Cost Routing selects lowest-cost matching route
Rate Application: Billing rate applied based on matched prefix
Step 4: Gateway Selection & Call Forwarding
Based on routing configuration, VOS3000 forwards the call:
Routing Gateway Prefix: According to VOS3000 manual, “when the number being called is not registered in the system, the call will be routed only to gateways which match the prefix specified”
Multiple Prefixes: Multiple prefixes can be specified, separated by commas
Gateway Priority: When multiple gateways match, selection follows priority, load balancing, and capacity rules
Step 5: Call Establishment
The terminating gateway processes the call:
100 Trying: Gateway acknowledges INVITE
180 Ringing: Destination phone starts ringing
200 OK: Call answered, SDP contains destination RTP information
ACK: VOS3000 confirms call establishment
Step 6: Media Stream (RTP)
After call establishment, audio flows between parties:
RTP Packets: Media flows between caller and called party
Media Proxy: VOS3000 can proxy media (configured per gateway)
Codec Negotiation: Final codec based on SDP negotiation
Step 7: Call Termination & CDR Creation
When the call ends:
BYE Request: Either party can initiate termination
200 OK: Confirmation of termination
CDR Record: Call Detail Record created with duration, cost, and status
Check the CDR (Call Detail Record) in Data Query section. The “Call End Reason” field shows why the call terminated. Use this to identify routing, authentication, or timeout issues.
Why are calls going to the wrong gateway?
Check routing gateway prefix configuration. VOS3000 routes based on prefix matching. Verify the gateway prefix matches your destination numbers and check gateway priority settings.
How do I fix one-way audio?
One-way audio is typically caused by NAT/firewall issues. Enable media proxy in gateway settings, ensure RTP ports are open, and configure NAT keepalive. See our RTP Media Troubleshooting guide.
What causes high PDD (Post Dial Delay)?
High PDD can be caused by network latency, slow gateway response, or DNS resolution delays. Check network quality, gateway performance, and consider using IP addresses instead of hostnames.
How can I improve ASR?
Analyze failed calls in CDR, identify common failure reasons, optimize routing paths, remove failing gateways, and ensure proper timeout configurations. Monitor gateway performance regularly.
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