Hi everyone,

I'm a Tello user living in Japan, and I recently discovered something interesting — and potentially concerning — while investigating how Tello handles VoIP and Wi-Fi Calling traffic.

I was curious about the network infrastructure used by US mobile carriers when accessing their services from overseas, so I decided to capture and analyze the traffic myself using Wireshark.

During a voice call made through the My Tello app on my Google Pixel 11, I noticed something unexpected.

My test setup

• Devices: Google Pixel 11 (Android) and a laptop running Windows 11 Pro
• Network: The Pixel was connected to a Windows Mobile Hotspot hosted by the laptop, which was connected to my home broadband connection in Japan
• Packet capture: Wireshark running on the Windows laptop
• Test call: An outbound call to a Japanese landline number providing a publicly accessible telephone-based radio broadcast service.

What I discovered

1. VoIP server connection

During the call, I observed continuous bidirectional UDP traffic between my device and 63.251.74.24, an IP address that appears to be associated with KeepCalling, the company behind Tello.

2. RTP traffic and audio codec

The UDP packets were approximately 214 bytes each, with packets transmitted at roughly 20 ms intervals.

Wireshark also identified RTCP traffic. After manually configuring the relevant UDP traffic to be decoded as RTP, Wireshark detected RTP streams using G.711 μ-law (PCMU).

The packet sizes and intervals were consistent with standard G.711 audio transmission.

3. Call audio could be played back without decryption

This was the most surprising part.

Using Wireshark's RTP stream analysis and playback features, I was able to reconstruct and listen to the actual received call audio directly from the captured packets, without using any decryption keys.

The recovered audio clearly matched the radio broadcast I was listening to during the call.

In other words, the audio was not merely identifiable as RTP traffic — the actual voice content was recoverable from the packet capture.

Why this concerns me

Based on these observations, it strongly appears that the My Tello app was transmitting received call audio using unencrypted RTP rather than SRTP, at least during my test.

If this is the application's normal behavior, someone with access to the relevant network traffic could potentially intercept and reconstruct call audio.

I understand that this does not automatically mean anyone on the internet can listen to these calls. However, transmitting voice audio without transport encryption would still raise legitimate security and privacy concerns.

Important clarification: My Tello app vs. native Wi-Fi Calling

This finding relates specifically to the VoIP calling feature inside the My Tello app.

It does not refer to Tello's native Wi-Fi Calling functionality through Android's standard phone dialer.

I separately investigated native Tello Wi-Fi Calling and observed IPsec-encrypted communication with T-Mobile infrastructure.

These appear to use different communication mechanisms, so I don't want to confuse the two.

Questions for the community

• Has anyone else captured and analyzed VoIP traffic from the My Tello app?
• Is unencrypted RTP an expected part of its current implementation?
• Does My Tello support SRTP or any other form of media encryption?
• Has anyone observed different behavior on other Android devices or on iOS?

I haven't shared the raw PCAP file because it may contain sensitive information.

This is based on my own observations from a single test environment, and I'm not claiming that every version of the app or every call necessarily behaves the same way.

I'd be interested to hear from other Tello users, network engineers, or anyone familiar with the app's VoIP implementation.

Thanks for reading!

I have frustrating and exasperating issues with Tello services. Constant call drops, disconnect, no network at home but not elsewhere. Phone is a Samsung Galaxy A14 5G.

For a few years I have Tello, this year 2026 I experience increasingly call drops, disconnects, or no connection or error messages displayed like "your phone is not on a network". Phone settings APN etc. are all correct.

Before Tello I had overpriced Verizon with a different and later the same A14 5G phone with no significant such issues, only occasionally.

Issues are: Signal strength suddenly drops to zero, no network (right now 11:10am Oct. 18 2026, as I write this message on my desktop) then suddenly I have 5 bars again full signal strength. Suddenly zero or an error message "your phone is not connected to any network. a few minutes later I have 5 bars signal strength again. then ~10 minutes later it drops to 4, sometimes to 3. and suddenly, randomly it's zero again. Oddly it appears, as the last few years solar disturbances have faded away it seems the issues increased.

This issue is most prominent at home (10 Gardenside Dr. San Francisco, CA 94131). if I go outside or anywhere else I have no issue calling receiving or SMS. Also, all the endless spam and scam calls never seem to have a connection problem haranguing me.

In fact I recently was in a concrete building in the garage 2nd subfloor underground with a low signal strength of two bars and testing. I had no issue sending receiving SMS or making phone calls.

SMS also exhibits same issue. Send a simple text message and I get a red error text "Failed to send. Tap for more options". Resend fails 2nd or 3rd time then suddenly goes through instantly.

Sometimes if I switch from network mode 5G to LTE it goes through, sometimes if I'm on LTE and switch to 5G it goes through. Then it seems to work in LTE mode but suddenly fails again a day or 2 later. switching back to 5G mode everything works smoothly again and vice versa.

Of particular interest is that it is more likely that calls/SMS get dropped with people who have at their end T-Mobile as service provider.

Of course in the traditional and usual blame game, everybody says it's my phone and wants to sell me a new phone but has no competent answer why then, if the issue were really my phone, why then does it work without a glitch in other locations!

As I write this message on October 18 11:20 AM signal strength suddenly and randomly fades out, drops to zero "no network connection" only to come back again later with 5 bar signal strength. SMS even with an animated GIF go swiftly through or suddenly not at all. Switch network mode to LTE it goes through. Another later message sent doesn't go through. switch network mode back to 5G, message goes sometimes right away through.

It's like playing darts trying to hit exactly the center of the black spot.

Also, how reliabel is SMS outside of cell coverage area (wifi internet connection only) ?

Thank you very much!