using FreePBX 188.8.131.52 pjsip outbound trunk, I did not find a way to dial out #21#.
Leading hash sign seems to be eaten and not make it through the trunk.
Tried multiple ways (dialing through custom feature code, etc.) and adding a custom dial plan in the trunk or outbound route (compose 21# and prepend #), etc. Nothing seems to work.
In contrast, dialing *21x.# through the trunk works well with setting up an outbound route dial plan (compose 21x.# and prepend *).
This question is addressed here, as I did not find any helpful answers in the freepbx community forum.