Configuring BSNL SIP Trunk (SIP PRI) on Asterisk with OpenVPN | A Complete PJSIP Guide (2026 Edition)

Q: What does this article say about 🧩 Step 2 – Configure BSNL SIP Trunk (PJSIP – NOT chan_sip)?

Modern Asterisk uses PJSIP , not sip.conf.

Q: What does this article say about 🔐 AUTH Section?

[bsnl_auth] type=auth auth_type=userpass username=+91XXXXXXXXXX password=yourpassword 📍 AOR Section If REGISTER trunk:

Q: What does this article say about ☎ 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 time

Q: What does this article say about 🔁 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

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 🚀

If you're exploring similar solutions or want to implement this in your business, I’m open to a quick discussion. Let’s evaluate your use case and identify the most efficient approach to get results.