Guest Users cannot access Microsoft Teams and External Vs Guest Sharing in Teams

An issue was reported below and explained subsequently,

Few users that were invited as guest in Teams to an external Organization. When they try to join Teams with their usual internal UPN they can't. They have to join as guest using the email they were invited by, which is different than their Microsoft 365 UPN. Customer have users that can communicate with external organization with M365 UPN (they are not invited as guest although). What are the possibilities and workarounds.

Explanation:

Reported issue is a default behavior for guest access i.e guests with their Organization's Teams access ID other than their email address, would have to use the email address, if they require to use the Teams environment of inviting party, to which they were invited. It has been already discussed that Customer's organization has external sharing enabled for that external organization and user are able to chat and meet with that organization. Mentioned below are some differences in external vs guest access in Microsoft Teams,

Feature	External Access	Guest Access
User can chat with someone in another company	Yes	Yes
User can call someone in another company	Yes	Yes
User can see if someone from another company is available for call or chat	Yes	Yes1
User can search for users across external tenants	Yes2	No
User can share files	No	Yes
User can access Teams resources	No	Yes
User can be added to a group chat	No	Yes
User can be added to a meeting	Yes	Yes
Additional users can be added to a chat with an external user	No3	N/A
User is identified as an external party	Yes	Yes
Presence is displayed	Yes	Yes
Out of office message is shown	No	Yes
Individual user can be blocked	No	No
@mentions are supported	No	Yes
Make private calls	Yes	Yes
Allow IP video	Yes	Yes
Screen sharing mode	No	Yes
Allow meet now	No	Yes
Edit sent messages	No	Yes
Can delete sent messages	No	Yes
Use Giphy in conversation	No	Yes
Use memes in conversation	No	Yes
Use stickers in conversation	No	Yes

Note: The above table is obtained from open discussions may not be considered as Microsoft Teams's Service Description

In reference to above discussion, if guest access is not an specific requirement, we would suggest to use the existing external sharing relationship and use the organization Teams ID to collaborate with external organizations. Furthermore, mentioned below is UserVoice link for feature to use Teams as guest with the Organization ID i.e other than invited email address.

https://microsoftteams.uservoice.com/forums/555103-public/suggestions/33799987-solution-for-guests-accounts-with-an-organisation