Unable to send messages via VoWifi

[TNZ9]

Checklist

• I can reproduce the bug with the latest version given here.
• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• This issue contains only one bug.
• I have read and understood the contribution guidelines.

Affected app version

1.8.0

Affected Android/Custom ROM version

Android 13

Affected device model

Gigaset GX4

How did you install the app?

F-Droid / IzzyOnDroid

Steps to reproduce the bug

1. Configure VoWifi on your phone
2. Turn on airplane mode or go somewhere with no cell signal
3. Connect to a WiFi network and make sure that the VoWifi service is working (call someone or your voicemail box)
4. Ask someone to send you a SMS text
5. If you received it, try to send one back

Expected behavior

The text messages I send should get sent successfully since I am connected to my service provider via VoWifi and since I am able to call people and receive SMS texts with that service.

Actual behavior

The message fails to be sent with an error stating an error code -1

Screenshots/Screen recordings

No response

Additional information

Being able to fully use VoWifi SMS would be great since I have some friends whose houses are not covered at all by mobile service providers and receiving messages without being able to reply is quite frustrating.
Please do not hesitate to ask for additional information. Since I don't know what would be the most useful.