Merge calls is not working

See this paragraph:

Start a conference call

With GSM, you can set up a conference call with up to five people (depending on your carrier).

Note: Conference calls may not be available if your call is using VoLTE (Voice over LTE) or Wi-Fi Calling.

1. While on a call, tap Add Call, make another call, then Tap Merge Calls.
2. Repeat to add more people to the conference.
3. During the conference call, do any of the following:

• • Talk privately with one person: Tap "i" in circle, then tap Private next to the person. Tap Merge Calls to resume the conference.
  • Add an incoming caller on the same line: Tap Hold Call + Answer, then tap Merge Calls.
  • Drop one person: Tap "i" in circle next to a person, then tap End.

Extracted from:--> https://support.apple.com/guide/iphone/while-on-a-call-iph3c9951d7/ios

Regards

Giulio