Configuring BSNL SIP Trunk (SIP PRI) on Asterisk with OpenVPN | A Complete PJSIP Guide (2026 Edition)
A practical, production-ready guide to configuring BSNL SIP PRI (IMS SIP Trunk) on modern Asterisk (PJSIP) with OpenVPN. Covers authentication, routing, user_eq_phone, scoring logic, debugging, and real-world carrier behavior.
BSNL now delivers SIP PRI over FTTH fiber using IMS-based SIP signaling routed through VPN (OpenVPN or SoftEther).
Key realities:
- SIP Proxy is reachable only via VPN
- SIP authentication may use:
- IMS expects proper user=phone format
- RTP may traverse different subnets
If your PBX doesn’t support OpenVPN natively, you must deploy a router or gateway device that does.
🛠 Step 1 – Configure OpenVPN
BSNL provides:
VPN Server Primary IP
VPN Server Secondary IP
Virtual IP (Client)
Gateway
Mask After connecting:
ip addr show tun0
ip route
ping 10.x.x.x # SIP Proxy Ensure:
- VPN interface is UP
- Route to SIP proxy goes through VPN
- SIP proxy is reachable
🧩 Step 2 – Configure BSNL SIP Trunk (PJSIP – NOT chan_sip)
Modern Asterisk uses PJSIP, not sip.conf.
Below is a production-ready configuration.
🔐 AUTH Section
[bsnl_auth]
type=auth
auth_type=userpass
username=+91XXXXXXXXXX
password=yourpassword 📍 AOR Section
If REGISTER trunk:
[bsnl_aor]
type=aor
max_contacts=1
qualify_frequency=30 If IP-auth trunk:
contact=sip:10.19.15.1:5060 ☎ Endpoint Section (CRITICAL)
[bsnl_endpoint]
type=endpoint
transport=transport-udp
context=from-trunk
disallow=all
allow=ulaw,alaw
auth=bsnl_auth
aors=bsnl_aor
from_user=+91XXXXXXXXXX
from_domain=upe.stbi.ims.bsnl.in
outbound_proxy=sip:10.19.15.1:5060;lr
user_eq_phone=yes
rtp_symmetric=yes
force_rport=yes
rewrite_contact=yes
dtmf_mode=rfc4733
timers=yes 🔎 Why user_eq_phone=yes Is Mandatory for India/BSNL
IMS expects:
sip:+91XXXXXXXXXX@domain;user=phone Without user_eq_phone=yes, calls may fail even if registration succeeds.
🔁 Registration Block (If Required)
[reg_bsnl]
type=registration
outbound_auth=bsnl_auth
server_uri=sip:upe.stbi.ims.bsnl.in
client_uri=sip:+91XXXXXXXXXX@upe.stbi.ims.bsnl.in
contact_user=+91XXXXXXXXXX
retry_interval=60
expiration=120 📞 Dialplan Configuration
Inbound
[from-trunk]
exten => +91XXXXXXXXXX,1,NoOp(Incoming BSNL Call)
same => n,Dial(PJSIP/1001) Outbound
[outbound]
exten => _9.,1,NoOp(Outgoing via BSNL)
same => n,Dial(PJSIP/${EXTEN:1}@bsnl_endpoint) 🎵 Codec & Media Recommendations
✔ Use only:
ulaw
alaw ✔ DTMF:
rfc4733 Avoid transcoding unless absolutely required.
🔍 Debugging & Troubleshooting
Enable SIP debugging:
asterisk -rvvv
pjsip set logger on
pjsip show registrations
pjsip show endpoint bsnl_endpoint
pjsip show contacts Network-level debugging:
tcpdump -i tun0 port 5060 📊 Carrier-Grade Monitoring Best Practice
In production systems:
- Don’t rely only on REGISTER
- Score trunk health using:
- Avoid marking trunk DOWN on single failure
- Use grace period logic
This avoids false negatives in telecom environments.
⚠ Common BSNL SIP Issues
ProblemCauseRegistered but no outgoing callsMissing user_eq_phone=yes403 ForbiddenWrong From headerNo audioRTP routing via wrong interfaceFlapping trunkVPN route instabilityWorks after reload onlyIncorrect AOR / section ID mismatch
🧠 Final Thoughts
BSNL SIP PRI is not “just another SIP trunk.” It behaves like IMS telecom infrastructure.
To deploy it correctly:
- Understand VPN routing
- Use proper PJSIP sections
- Align endpoint IDs correctly
- Format Indian numbers properly
- Monitor with carrier-grade logic
When configured properly, it is extremely stable.
If you're building multi-tenant PBX systems, telecom billing engines, or AI-driven calling platforms, mastering IMS-style SIP trunks is essential.
Happy Building 🚀