There are times where a one to one call is just not yielding the right answers and you need to drag someone into the call to help gain a sufficient answer.
Microsoft Teams supports multi-party calling to give its official term. However, there are some limitations on its use you need to be aware of.
The rules on who you can add or cannot add are dependent on the original call type.
If the call is between yourself and someone who has dialled your phone number from another phone system, e.g their mobile, then you can drag in your coworkers to the conversation.
If the call is between yourself and a coworker, you can drag other coworkers in to the conversation and one federated contact.
If the call is between yourself and a federated contact, then you cannot drag anyone else into that call currently. You must hang up and call them back after first calling your coworkers.
Drag A Person Into Your Call in Microsoft Teams
When in a call you will notice an icon of two people. This signifies the drag in feature

When clicked it will open a side blade and ask you to invite someone or dial a number.

You can search for a coworker using their name, or a federated contact using their e-mail address.
Calling a number depends if you are enabled for Audio Conferencing Dial out and have a calling plan with Microsoft. You will get a status update of the invitation.

Remove a Participant from the Call
If someone does not answer, maybe you get their voicemail, you can remove them from the call by clicking on the ellipsis next to their identity and selecting Remove from Meeting
