Unable to see Free Busy through Exchange Federation

You receive following error while fetching free Busy for another Federated Organization and get following error from Exchange On-perm servers.

[PS] C:\>Test-FederationTrust
RunspaceId : 49c3c312-1a26-4cd8-a14d-59d736458f76
Id         : FederationTrustConfiguration
Type       : Success
Message    : FederationTrust object in ActiveDirectory is valid.

RunspaceId : 49c3c312-1a26-4cd8-a14d-59d736458f76
Id         : FederationMetadata
Type       : Error
Message    : Unable to retrieve federation metadata from the security token service.

RunspaceId : 49c3c312-1a26-4cd8-a14d-59d736458f76
Id         : StsCertificate
Type       : Success
Message    : Valid certificate referenced by property TokenIssuerCertificate in the FederationTrust object.

RunspaceId : 49c3c312-1a26-4cd8-a14d-59d736458f76
Id         : StsPreviousCertificate
Type       : Success
Message    : Valid certificate referenced by property TokenIssuerPrevCertificate in the FederationTrust object.

RunspaceId : 49c3c312-1a26-4cd8-a14d-59d736458f76
Id         : OrganizationCertificate
Type       : Success
Message    : Valid certificate referenced by property OrgPrivCertificate in the FederationTrust object.

RunspaceId : 49c3c312-1a26-4cd8-a14d-59d736458f76
Id         : TokenRequest
Type       : Error
Message    : Failed to request delegation token.

Resolution:

You need to make sure Exchange On-Prem servers are able to reach MFG i.e there is no as such restrictions.
You need to check if any stale InternetWebProxy attribute is configured at the Exchange servers level.