PSTN call transfers and callpark on a Become silent after in a direct routing environment.

Are you using direct routing, or calling plans? I've seen this happen in direct routing environments where the reINVITE message is being sent, and one side doesn't support it. Depending on the SBC, there is usually some good documentation on configurations for Teams setup. For example AudioCodes has a document that walks through all the appropriate settings for their SBC line.

*EDIT* I should have read a little closer, I do see you're using direct routing.  I'd double check the configurations for handling reINVITE messages.  For example, AudioCodes suggest having it configured to "handle locally", which I think believe the SBC from telling Teams that it supports it, and helps with this issue.