Guest is unable to join teams meeting without sign in

sj485 Hello, I have no experience from Graph. But I do know about Teams and how to set it up for anyone to join so hope this helps.

There's a general setting called "anonymous join" https://docs.microsoft.com/en-us/microsoftteams/meeting-settings-in-teams#allow-anonymous-users-to-join-meetings 

And you also have the lobby settings called AutoAdmittedUsers, can be set per-meeting or by using policy. I noticed in the text above that it said EveryoneInCompany and that would explain why external cannot join directly https://docs.microsoft.com/en-us/microsoftteams/meeting-policies-in-teams#automatically-admit-people

-AutoAdmittedUsers
Determines what types of participants will automatically be added to meetings organized by this user. Possible values are:

EveryoneInCompany, if you would like meetings to place every external user in the lobby but allow all users in the company to join the meeting immediately.

Everyone, if you'd like to admit anonymous users by default.

EveryoneInSameAndFederatedCompany, if you would like meetings to allow federated users to join like your company's users, but place all other external users in a lobby.

OrganizerOnly, if you would like that only meeting organizers can bypass the lobby.

This setting also applies to participants joining via a PSTN device (i.e. a traditional phone).