#Connection troubleshooting

If registration, calling or SMS fails, find the matching section below.

#ePDG does not respond or times out

• Make sure your network is inside the country or region of the SIM card; failing to register while abroad is a normal restriction.
• Public Wi-Fi often blocks IPsec (UDP 500 / 4500); switch to a personal hotspot or another stable network and retry.
• Avoid hammering retries: wait 1-2 minutes between attempts, bursts can trigger a temporary carrier limit.
• If it still fails, open Advanced Settings -> View log and check the stuck stage ([220] / [195] means the ePDG did not respond).

#IMS registration rejected

• Confirm the carrier has VoWiFi / Wi-Fi Calling enabled for this SIM.
• Make sure you are on a network inside the SIM's country or region.
• In Advanced Settings -> View log check the last IMS SIP status= value; 403 / 404 need confirmation from the carrier.

#Call fails

• Make sure the number is valid on this carrier's network; short codes (e.g. 40430) require the home country network.
• Dial international numbers with the country code (e.g. +86...).
• Service short codes (e.g. 40430): most carriers (including O2/giffgaff) do not route short codes over Wi-Fi Calling — both URI shapes get terminated by the network. To dial a short code, move the SIM to a phone and use the cellular network.
• Switch to a more stable network, re-register and try again.
• Use Advanced Settings -> Call experiments to compare INVITE shape / TCP inside the tunnel / QoS preconditions.

#SMS cannot be sent or is not delivered

• "Missing SMSC": set the service-centre number in Connection -> Advanced settings; giffgaff / O2 defaults to +447802002606.
• Stuck at "accepted": the network is delivering; the status report usually returns within seconds to minutes.
• Not delivered for a long time: verify the recipient number; international numbers need the country code.

#Audio problems

• Allow SimCat to use the microphone in system settings.
• After switching Bluetooth / wired headsets, end the call and dial again.
• Audio dropouts usually mean the route changed; keep the app in the foreground and call again.

#Network requirements

• Wi-Fi Calling only works on networks inside the SIM card's country or region.
• Firewalls must allow UDP 500 and UDP 4500.
• Corporate / campus / hotel networks often block IPsec; test on another network.