TL;DR: Galaxy S7 (Android 8) can't call Pixel 8a (GrapheneOS), but reverse direction works fine. Galaxy S7 can call other modern phones (iPhone, landlines) without issues. SMS to Pixel 8a work fine. Problem seems specific to the combination of S7 + GrapheneOS. Research indicates this is a VoLTE codec incompatibility issue with the outdated S7, not a GrapheneOS problem.
Hi everyone,
I'm experiencing a strange one-way calling issue between two of my phones and hoping someone here might have insights.
Setup:
Galaxy S7 running Samsung Android 8 (no longer updateable) with Google Family Link installed
Pixel 8a running GrapheneOS (fully updated system and apps)
Problem:
I cannot call from the Galaxy S7 to the Pixel 8a. The calls simply don't go through. However, calling from the Pixel 8a to the Galaxy S7 works perfectly fine.
What works:
The Galaxy S7 can call other modern phones (like my wife's iPhone) and landlines without any problems
SMS from the Galaxy S7 to the Pixel 8a work fine
When I put the S7's SIM into any other phone, I can reach the Pixel 8a just fine
It's specifically voice calls from S7 to Pixel 8a that fail
Important observation:
The problem seems to be specific to this exact combination of the Galaxy S7 hardware/software and GrapheneOS. When I use the same SIM in a different, more modern phone (also with Family Link installed), I can call the Pixel 8a without issues. So I don't think Family Link is the culprit.
What I've tried:
Swapped SIM cards between the devices - no change, so it's not SIM-related
Checked blocked numbers list - S7 number is not blocked
Tried various network settings (though I'm not very confident with those)
Rebooted both devices multiple times
Switched the phone app on the Pixel to Google Phone - no effect
Updated all apps on the Pixel 8a and ensured GrapheneOS is fully up to date
Factory reset the S7 (apps are freshly installed)
Research findings (Claude):
After doing extensive research into GrapheneOS forums, GitHub issues, and VoLTE troubleshooting documentation, I found that this is a known VoLTE compatibility issue:
The Galaxy S7's VoLTE implementation from 2016 uses outdated codec negotiation. When the S7 initiates a call, it sends old-style SIP parameters that modern devices may reject. The reverse works because modern devices like the Pixel 8a are designed to be backward-compatible when initiating calls to legacy endpoints.
GrapheneOS uses identical carrier configurations to stock Pixel OS and makes no modifications to telephony protocols. No similar issues have been reported in GrapheneOS communities specifically - this points to the S7's aging VoLTE stack as the problem.
The Samsung Galaxy S7 had documented VoLTE issues after the Android 8 Oreo update, with many users reporting the VoLTE option disappearing entirely or becoming unreliable.
Likely solution:
Disabling VoLTE on the Galaxy S7 should resolve this. On Samsung devices: Settings → Connections → Mobile Networks → VoLTE calls → Turn off. This forces the phone to use legacy circuit-switched calling if available.
Alternatively, the permanent solution is replacing the 7-year-old S7 with a more current device (Android 10+).
Has anyone experienced something similar or have any other suggestions?
Thanks for any help!