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🌍 Using Telnyx + Zoiper for Canadian PSTN Calls Anywhere

· 3 min read
Frank Chen
Frank Chen
Backend & Applied ML Engineer

I am developing a voice agent outside Canada. Without my Canadian SIM, staying connected via PSTN means I rely on VoIP softphones and Telnyx SIP trunking. In this guide, I’ll walk through how I set up two Canadian Telnyx numbers and used Zoiper on macOS and iOS to call between them—without needing PSTN compliance.

⚙️ 1. Telnyx Configuration​

🔐 Credential-Based SIP Connections​

I created two separate credential-based SIP Connections in the Telnyx console, each bound to a Canadian DID:

{
  "sip_trunk_connections": [
    {
      "connection_name": "livekit-trunk",
      "connection_type": "credential",
      "domain": "sip.telnyx.com",
      "username": "userfrankchen9081",
      "password": "YJhY0pg0",
      "numbers": ["+12495304098"]
    },
    {
      "connection_name": "originmind-callee",
      "connection_type": "credential",
      "domain": "sip.telnyx.com",
      "username": "userfrankchen8081",
      "password": "YJhY0pg0",
      "numbers": ["+12495304046"]
    }
  ],
  "telnyx_test_center_number": "+18004377950"
}

Each connection uses Credentials authentication which allows Zoiper to register and handle both inbound and outbound calls. 📘 Telnyx docs note this is ideal for softphones needing registration and dialing ([Quickstart guide]) (developers.telnyx.com, telnyx.com).

📞 Outbound Voice Profile​

To enable outbound PSTN calls, I created an Outbound Voice Profile and attached it to each connection—allowing both Zoiper clients to place calls.

🖥️ 2. Zoiper Softphone Setup​

macOS (Number A) • Configured using sip.telnyx.com and the credentials above. • Adjusted STUN and rport settings for stable connectivity.

iOS (Number B) • Similar config, but change the following if encountering connecting problems: • Disabling STUN • Toggling rport • Proper codec selection • Noted that background registration on mobile often fails unless these settings are correct.

🔁 3. Testing Call Flows​

• Outbound Call Testing: I called +1-800-437-7950 (Telnyx test center) to confirm outbound was functioning. • Call Between Softphones: I made calls: • macOS ➜ iOS • iOS ➜ macOS All calls connected successfully.

I used Telnyx’s SIP Call Flow debugging tool to inspect INVITEs, media negotiation, and responses—an invaluable resource for troubleshooting.

🌐 4. Hybrid Mode: FQDN + Credentials (Clarified)​

⚠️ Important Correction: With a single SIP Connection set to FQDN + Credentials, inbound PSTN calls go exclusively to your FQDN endpoint, not to registered devices. Registered credentials only serve SIP-URI and outbound calls. 📘 Official docs confirm: FQDN handles inbound routing, credentials handle registration/outbound only (developers.telnyx.com, docs.livekit.io, support.telnyx.com).

💡 5. Lessons Learned​

• Use credential-based trunks when testing with softphones. • On iOS Zoiper, disable STUN and tweak rport for reliable registration. • Leverage Telnyx debugging tools (SIP Call Flow, PCAP export) for diagnostics. • Don’t expect DID calls to reach registered clients if the connection is FQDN+credentials only. For that, stick to credentials-only or add your own proxy/forking logic.

🎯 6. Conclusion​

With Telnyx + Zoiper, you can simulate two-way Canadian PSTN calling from anywhere in the world—even without a local SIM. Credential-based SIP trunks enable full bi-directional calling through Canadian DIDs. Adding hybrid FQDN setups is powerful, but only for directing DID calls to your infrastructure—not softphones.