BASIC INFORMATION
Please provide a descriptive title for your feedback:
Unable to make/receive Wi-Fi Calls with
WINDTRE mobile carrier and Sky Wifi landlines (MTU issues)
Which area are you seeing an issue with?
Cellular Service (Calls & Data)
What type of feedback are you reporting?
Incorrect/Unexpected Behavior
DETAILS
The Enhanced Beta Feedback feature helps
Apple diagnose telephony and cellular service issues. You can learn more and optionally enable this feature in Settings > Privacy > Analytics & Improvements Have you enabled this feature?
Yes
What does the cellular issue you are seeing involve?
Calls
Is your iOS device currently in the problem state?
If yes, please answer the following questions and be sure to "Gather" the iOS Baseband Logs.
If no, please reproduce the problem and leave the device in the problem state, then create a new feedback.
Yes
Which cellular network (Carrier) is active on your device? (see
Settings>General>About>Network) If more than one, please list both.
WINDTRE 56.5.1
Are you using an eSIM?
No
Which city were you in when the issue occurred?
Ceprano, FR, Italy
What region is your device set to? (see Settings > General > Language & Region)
Italy
What was the specific time when this occurred? We need this to pinpoint your issue in the logs.
21:25:00 01 Nov 2023 (local time)
DESCRIPTION
Please describe the issue and what steps we can take to reproduce it:
Description of the problem:
Wi-Fi Calling in Italy has just been launched with this developer beta release (iOS 17.2 beta, WINDTRE 56.5.1 carrier bundle) but, with WINDTRE carrier, it is faulty and gives an unacceptable behaviour when iPhone is connected to any Wi-Fi network attached to a landline from "Sky Wifi" (Sky Italia, NBC-Comcast group) provider (tried on a couple of different lines).
Outgoing calls are mute and, after a very long timeout, they are sent via mobile network, but, if mobile signal is weak or absent, you get a call error and call is not established.
Incoming calls are received only if a good mobile signal is present, otherwise they are sent to the voicemail, despite the "WINDTRE WiFi Call" logo is displayed.
The actual behaviour is unacceptable and far from Apple standards: the user can think to be fully reachable as "WINDTRE WiFi Call" logo is shown but actually he is not, especially when mobile signal is weak or absent.
When using a SIM card from "TIM" Italian mobile carrier, which launched the service with the same iOS beta version, everything works perfectly on the same Wi-Fi network, so this issue is certainly related to a bad parameter inside WINDTRE carrier configuration (spoiler: it is UseNetworkMTU).
With WINDTRE, the IPSec/KEv2 tunnel is correctly established and IMS registration is fine, but some larger packets during calls are missing (see attached image with WireShark analysis on the WAN interface). From my testing, the unexpected behaviour is certainly related to an incorrect MTU value inside the IPSec/KEv2 tunnel.
Sky Wifi is probably the first big operator worldwide using an IPv6-only core network and the MAP-T translation protocol for IPv4, therefore this issue has never been noted before. MAP-T protocol implies fragmentation issues with any IPSec/IKEv2 VPN too if MTU size is not set correctly! The web is full of reports but this cannot be solved easily by Sky. From my many tests, I can say the maximum allowed MTU size for the virtual interface is 1330 bytes. Please read below for details.
A quick solution, would be if you can remove the "UseNetworkMTU" flag from the "ims" apn settings for WINDTRE (with TIM it is not necessary and it works like a charm) or if you can add something else to reduce the virtual interface MTU size when the IMS tunnel is created.
Step-by-step:
- Insert a WINDTRE (22288) sim card (with VoLTE/VoWiFi service enabled) and connect to a Wi-Fi network from "Sky Wifi" Italian landline operator or another one using an IPv6-only core network and MAP-T translation protocol for IPv4. Router is the default one provided by the latter (model SR203) and has very basic settings (nothing a user can change has impact on this issue) and AP/Wi-Fi signal is excellent
- Enable Wi-Fi Calling feature in the Settings
- 4G mobile signal should be low enough (due to "ims" iRatPolicy)
- Wait a few seconds until Wi-Fi Calling is up
("WINDTRE WiFi Call" is displayed)
- Make an outgoing call to any regular Italian number (I tried to call +390775914682 at 21:26:00)
- iPhone remains mute while trying to establish call without success and, after about 20 sec timeout, the call is routed via the mobile network (if mobile signal is enough, otherwise call error) and "WINDTRE" regular logo is shown until the call ends
- Immediately after the end of the call, the
"WINDTRE WiFi Call" logo is back
- Try to receive an incoming call (in my log a call has been received around 21:27 from my home number +390775914682)
- Wi-Fi Calling will be paused for the duration of the call ("WINDTRE" regular logo is shown) and the call goes through the mobile network
- Wi-Fi Calling logo ("WINDTRE WiFi Call") will be back immediately after hang-up
Result actually saw:
Outgoing calls:
When Wi-Fi Calling is enabled and connected
("WINDTRE WiFi Calls" operator name is shown), outgoing calls are mute until 20 seconds timeout, then they are rerouted via mobile network if signal is enough, otherwise the phone gives Call Error.
Incoming calls:
Calls are received via mobile network
("WINDTRE" operator name is sho
(continued)
wn) if mobile signal is enough, otherwise call is missed and diverted to voicemail.
Expected behaviour:
Wi-Fi Calls are made/received without any issue through any Wi-Fi network, including Sky Wifi landlines, as it happens with TIM mobile carrier on the same iOS version. Maximum compatibility with any network should be accounted when deploying this very important feature. In Italy indoor mobile reception is very bad due to thick walls, mountains and extremely low electromagnetic power limits.
Please help us solve this frustrating issue.
Thank you.
