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Alex susanu task product support engineer telnyx

**Customer Dilemma:

**A new Telnyx customer is attempting to set up their portal account and start making calls through a softphone. However, they’re having trouble getting the calls to complete to their cell phone. Based on the customers outreach below, how would you respond to the customer, as if you were working with them to solve their issue? 

"Hi there Telnyx support, I just set up a Telnyx account and followed your guide for setting it up with Zoiper 5, here.

I seem to have followed everything correctly, but I keep getting an error when I try to dial my cell phone number. I was able to get my tech guy to grab a call example in this pcap file, I am hoping that you’re able to dissect what the issue is and explain to me how I can fix it". (Please find the example pcap attached to this email also)

In order to best assist the customer with your response, please leverage the documentation available here. Besides, this will show you which IPs are form Telnyx. You can also leverage Google to find general information about SIP and about wireshark.

Your tasks:

  1. Explain how to view the SIP Flow sequence through wireshark (you should download this if you don’t already have it).
  2. Explain which IP is the customer’s and which IP is Telnyx’s - how did you know?
  3. In the example pcap, what number is dialed and what is currently attempted to be used as the "caller ID"?
  4. How would you format a response back to the customer explaining what is causing their calls to fail and what can the customer do to fix the issue? Please format this as a professional email to the customer.

1. SIP Flow in Wireshark

Wireshark v4.6.4

After uploading the pcap file (File -> Upload file), select Telephony -> VoIP Calls -> Flow Sequence ![[Screenshot 2026-04-09 at 19.09.49.png]]

![[Screenshot 2026-04-09 at 19.09.56.png]]

![[Screenshot 2026-04-09 at 19.10.04.png]]

![[Screenshot 2026-04-09 at 19.10.11.png]]

2. Customer IP vs Telenyx IP

Telenyx IPs are always the same, available at https://sip.telnyx.com/ ![[Screenshot 2026-04-09 at 17.11.40.png]]

src is the client's IP since they are the one initiating the call ![[Screenshot 2026-04-09 at 19.30.54.png]]

3. Number dialled vs Caller ID

trying to dial

to: (708) 595-2391 from called id: zach@sip.telnyx

![[Screenshot 2026-04-09 at 19.33.57.png]]

4. Email to customer

Hello,

Thank you for reaching out and for providing the packet capture, that was very helpful in troubleshooting faster.

After analysing the pcap file, I managed to replicate the issue as well. I can confirm the root cause: your calls are being rejected by Telnyx with a 403 error, "Caller Origination Number is Invalid."

When making calls Telnyx requires an outbound called ID to be a real number. Zoiper is not able to set an outbound caller ID, this change needs to be made in your Telnyx account.

To fix this, please follow these steps:

click on: Real time communications -> Voice -> SIP Trunking select the SIP connection (e.g. test) -> Edit -> Outbound tab -> Caller ID override (see below screenshots)

The caller ID override here is what will be displayed on the screen when calling someone this number needs to one of the Telnyx numbers you own and/or outside Telnyx but verified by you

In the example below I used a number ending 8752, as this is a number belonging to the test account

In your case use one of the numbers from your Telnyx account. there is a Numbers tab right next to it you can copy from.

![[Screenshot 2026-04-09 at 20.00.41.png]]

![[Screenshot 2026-04-09 at 19.59.07.png]]

![[Screenshot 2026-04-09 at 19.59.22.png]]

![[Screenshot 2026-04-09 at 19.17.09.png]]

Please let me know if you encounter further issues and if I can help in any way.

Thank you

Alex Susanu Product Support Engineer @ Telnyx