facing issues in calling and messages in microsoft teams with external users

A Teams or Global Administrator should enable both organization-level external access and the affected user’s external-access policy; however, direct Teams calls cannot be enabled for recipients using Teams Free or another unmanaged Microsoft account.

1. Identify the external account type

This determines which functionality is supported:

• Managed work or school Teams account: External access can support chat, calls, and meetings, provided both organizations allow the communication.
• Teams Free or unmanaged Microsoft account: External access supports chat, but not direct one-to-one Teams calling. Use a scheduled Teams meeting for audio/video, or have the recipient use a managed organizational Teams account.^1^
• Guest account in your tenant: Guest access—not external access—controls access to your teams and channels. These are separate configuration paths.^2^

2. Check organization-level external access

A Teams or Global Administrator should:

1. Open Teams admin center.
2. Go to Users > External access.
3. Under Teams and Skype for Business users in external organizations, select either:
   • Allow all external domains, or
   • Allow only specific external domains, then select Add external domains and add the recipient’s exact domain.
4. Confirm the domain is not in a blocked-domain list.
5. Select Done or Save.^1^

If an allow list is configured, every domain not explicitly listed is blocked. Also check subdomains separately because blocking a parent domain does not block its subdomains by default.

External access must be permitted on both sides: the recipient’s organization must trust your domain and enable its user for external access. If only one organization permits it, chat and calling can still fail.^1^

3. Check the affected user’s policy

Organization access alone is insufficient. The external-access policy assigned to the affected internal user must also permit external communication; organization settings can override the user policy.^1^

Do not use the deprecated CustomizeFederation parameter. Federation changes require Teams or Global Administrator permissions, and the current tenant federation configuration or external-access policies should be used instead.^3^

4. Check unmanaged and trial-only accounts

For an unmanaged Teams account, turn on:

1. Teams admin center > Users > External access.
2. Enable People in my organization can communicate with unmanaged Teams accounts.
3. To let unmanaged users initiate chats, also select External users with Teams accounts not managed by an organization can contact users in my organization.
4. Select Save.

This enables supported chat/meeting interactions, but does not add direct Teams calling to unmanaged accounts.^1^

If the external organization is a Teams trial-only tenant, communication is blocked by default. Using Microsoft Teams PowerShell 6.4.0 or later, an administrator can allow it:

# Run with Teams administrative permissions.
Set-CsTenantFederationConfiguration `
  -ExternalAccessWithTrialTenants "Allowed"

# Use "Blocked" to reverse the setting.

If both organizations are trial-only tenants, both must set this value to Allowed.^1^

5. Run the external-access diagnostic

An administrator should run the Teams external communication diagnostic in the Microsoft 365 admin center:

1. Open the diagnostic’s Run Tests pane.
2. Enter the affected user’s SIP address and the external organization’s domain.
3. Select Run Tests.
4. Apply the returned policy or domain-configuration recommendations.^1^

If administrator assistance is required, provide:

• Affected external email address and domain
• Whether the recipient uses managed Teams, Teams Free, or a trial-only tenant
• Exact error message or whether the call buttons are unavailable
• Whether every external contact fails or only one domain
• Whether chat, direct calls, and scheduled meetings fail independently

References

1. IT Admins - Manage external meetings and chat with people and organizations using Microsoft identities
2. Chat, teams, channels, & apps in Microsoft Teams
3. Teams External Access Settings Not Applying: CommunicationWithExternalOrgs and CustomizeFederation Issues - Microsoft Q&A