FreePBX allows blind transfer only

One of the installs I manage only seems to allow blind transfers of calls. Example: extension 1 calls extension 2, extension 2 then transfers the call to extension 3. When transferring blindly, the calls goes through, other wise when waiting for extension 3 to pick up, announcing the caller and pressing the transfer button, the extension still on hold gets the message the extension dialled can’t be reached and the call is dropped. All extensions are set to being allowed 2 calls (Max audio streams=2). I have tried the same flow with my own FreePBX where I don’t have this issue. Funny thing, Max audio streams is set to 1 on my server.

What could this issue be?

Outbound Concurrency Limit maybe ?
The value must be 2 (or more) for it to work.

Nope, that’s set to 3 for all extensions.

Show actual debugs of this happening. Show us what is going on, we can’t guess what is happening.