@mario ,
According to your description, your environment should belong to multiple deployments of Skype for Business Server in multiple forests.
The following key requirements should be met:
- There must be at most one Microsoft 365 or Office 365 organization involved.
- At any given time, only one on-premises Skype for Business forest can be in hybrid mode (shared SIP address space).
Then, follow this link to configure hybrid connectivity.
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