Call disconnected transferring to parked extension

The current setup I have is like this:

Call server with an outgoing IAX2 trunk with an outgoing route that matches 12345X. to a park server

The park server has custom code in extensions_custom_conf that looks like this:

exten => 12345300,hint,park:[email protected]
exten => _12345.,1,Answer
exten => _12345.,n,Set(PARKINGEXTEN="${EXTEN}")
exten => _12345.,n,ParkAndAnnounce(a,900,Console/dsp)
exten => _12345.,n,hangup

If a user transfers someone to extension 12345300 the system correctly parks them at that extension.

After the user parks the call at extension 12345300 they are able to dial this extension and get connected. After they do this, however, they may need to transfer them back to extension 12345300 in order to put them back on the parked extension.

When executing a blind transfer back to extension 12345300 the line automatically disconnects.

Any ideas on how to put the user back onto the parked extension?

Thanks!