How to Make and Receive SIP to PSTN Calls Using Twilio and Zoiper (Step-by-Step Setup Guide)

The fastest way to make calls using Twilio and Zoiper (or any other soft phone like Linphone) is to register your softphone directly to a Twilio SIP Domain. Twilio acts as the bridge between your SIP client and the PSTN.

This approach requires no PBX and is ideal for testing, prototypes, or for personal use especially if you are in international roaming.

Step 0: Set Up the Twilio Dashboard

In your Twilio Console dashboard, first pin the following products for quick access from the sidebar.

Click “Explore Products” in the sidebar and pin:

Voice
TwiML Bins
Phone Numbers

Your sidebar should now look similar to the screenshot shown below.

Step 1: Create SIP Credentials in Twilio

Go to:

Voice → Manage → Credential Lists

Create a new Credential List

A Friendly Name (anything descriptive)
A username
A strong password

You will use these credentials inside Zoiper.

Step 2: Create a TwiML Bin for Outbound Calls

Go to:

TwiML Bins → My TwiML bins

Create a new TwiML Bin configuration

Set a Friendly Name: anything descriptive
Paste the following XML:

			
<?xml version="1.0" encoding="UTF-8"?>
<Response>
  <Dial answerOnBridge="true" callerId="+14155551234">
    {{#e164}}{{To}}{{/e164}}
  </Dial>
</Response>

		

Replace +14155551234 with a Twilio number you own.

Why this matters

{{To}} forwards the number dialed from Zoiper
callerId ensures PSTN calls display your Twilio number
answerOnBridge="true" prevents early media issues

Click Create to save the TwiML Bin.

Step 3: Create a SIP Domain

Go to:

Voice → Manage → SIP Domains

Configure:

Friendly Name: anything descriptive
SIP URI: yourprefix.sip.twilio.com
Voice Authentication: select your Credential List
Enable SIP Registration: ✔

Under Call Control Configuration:

“A CALL COMES IN” → Select TwiML Bin

Save the configuration.

Step 4: Configure Zoiper (or Any Softphone)

Add a new SIP account:

			
Username: your_sip_username
Password: your_sip_password
Domain: yourprefix.sip.twilio.com
Transport: UDP (recommended for testing)

Once configured, Zoiper should show Registered
If it doesn’t:

Check credentials
Confirm SIP Registration is enabled
Verify domain spelling

Step 5: Make a Call

Dial in E.164 format:

			
+14155551234
+491701234567

Twilio expects the leading + and country code.
When you press call:

Zoiper sends SIP INVITE to Twilio
Twilio executes your TwiML
Twilio dials the PSTN number
The call bridges once answered

Step 6: Receiving Incoming Calls

Create a TwiML Bin for Inbound Calls
Go to:

Voice → TwiML Bins → Create New
Paste:

			
<?xml version="1.0" encoding="UTF-8"?>
<Response>
  <Dial>
    <Sip>sip:your_sip_username@yourprefix.sip.twilio.com</Sip>
  </Dial>
</Response>

		

Replace:

your_sip_username → Your SIP credential username
yourprefix → Your SIP domain prefix

Save the TwiML Bin.

Attach the TwiML Bin to Your Twilio Number
Go to:

Phone Numbers → Manage → Active Numbers → Your Number

Under A CALL COMES IN:

Select TwiML Bin
Choose the TwiML Bin you just created
Click Save

Now inbound calls will ring your registered softphone.

Common Issues and Fixes

403 Forbidden

Wrong credentials
SIP Registration not enabled

21217 Caller ID Error

CallerId in TwiML is not a Twilio-owned number

Call connects but no audio

NAT/firewall issue
Try UDP transport
Enable STUN (e.g., stun.l.google.com:19302)

When to Use Elastic SIP Trunking Instead

If you have a PBX (Asterisk, FreePBX, 3CX), use Twilio Elastic SIP Trunking instead.
In that case:

Twilio connects to your PBX
Zoiper registers to your PBX
Your PBX handles routing

Direct SIP registration is simpler. Trunking is better for complex call routing.

What Happens Behind the Scenes?

When you press dial:

Twilio becomes a programmable SIP server
Your TwiML defines how the call is routed
Twilio handles PSTN interconnect
Media flows between Zoiper and Twilio

You are essentially building your own lightweight phone system using Twilio’s programmable voice layer.

Final Checklist

Twilio number purchased
TwiML Bin created and saved
CallerId replaced with owned number
SIP Domain created
Credential List attached
SIP Registration enabled
Softphone shows “Registered”
Dialing in +E.164 format

Conclusion

Making calls with Twilio and Zoiper is straightforward once you understand the flow: SIP client → Twilio → PSTN.

For quick setups and prototypes, direct SIP registration is the pro move. It’s simple, flexible, and gives you full programmable control without running your own PBX.

References & further reading

Twilio Console (OpenAI Platform, OpenAI)
Twilio TwiML Bins (TwiML Bins)
Zoiper Download Page (Download Zoiper)
Download Linphone (Download Linphone)

Akash Gupta
Senior VoIP Engineer and AI Enthusiast

AI and VoIP Blog

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