@Enserr 

Hello, please help confirm details and we will try to figure it out:
1. Ensure the Deco M5 firmware is up-to-date. If not, update the firmware first on the Deco app.
2. Ensure the Deco app is the latest as well.
3. Please confirm the current network topology/layout, how many Deco M5 do you have and how are they connected? Is the iPhone 12 connected to the Deco 2.4GHz or 5GHz network? Try to connect it to the main Deco. BTW, who is the Wi-Fi Calling provider? Is the Deco working as a router or an access point?
4. Is there any other phone that you can try this Wi-Fi Calling on? Ensure Wi-Fi Calling is working fine when the phone is connected to other wireless networks.

@TP-Link_Deco 

hi there. I too am having this issue. iPhone 6S, iPhone 7 and iPhone 6 Plus all latest software version. 
deco setup is 5x M5 EU 3.0 acting in router mode with plusnet ADSL 36mb down 7mb up. Deco firmware 1.5.1 rel. 66804

wificalling either doesn't work or is intermittent with three network and EE network. 
 

what can I try?

You can check some settings on iPhone, like turning on wifi calling or resetting networking settings. If no use, you can use an ios repair tool like TunesKit to fix the issue with no data loss.

Hi @trucjacks123 and all, 

If you still encounter an issue with the Wi-Fi calling in the Deco network, please try the suggestions we provided here:

What should I do if Wi-Fi calling cannot work through TP-Link wireless router?

Additionally, you may try the suggestions in this thread as well:

[Troubleshooting] Zoom or Microsoft Teams frozen or hanging when connected to Deco - Home Network Community

Hope this helps, kindly get back with the details if it is not.