Wifi calling not working

WiFi calls is a Carrier Specific feature. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you. Please see the pic below which is self-explanatory...

Note: If the Wi-Fi connection is lost when you make calls, calls switch automatically to your carrier’s cellular network using VoLTE (Voice over LTE), if available and turned on. See View or change cellular data settings on iPhone. (VoLTE calls also switch to Wi-Fi when a Wi-Fi connection becomes available.) Contact your carrier for feature availability.

When your iPhone has a low cellular signal, use Wi-Fi Calling to make and receive calls through a Wi-Fi network.

1. On your iPhone, go to Settings > Cellular.
2. If your iPhone has Dual SIM, choose a line (below SIMs).
3. Tap Wi-Fi Calling, then turn on Wi-Fi Calling on This iPhone.

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Make calls using Wi-Fi on iPhone - Apple Support (IN)

Many users who have updated iOS must have a similar issue if the issues stated by you are due to the iOS.

Unless you state what the issue is many in this forum may not be able to respond positively to the issue you are facing. 

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.