Krish & Pix,
BSC will never use FACCH call set up if sdcch is available. If there is no SDCCH available BSC tries to allocate FACCH for call set up signalling instead of rejecting the call.
So both SDCCH + TCH congestion will cause paging failure.
Also I think FACCH call set up is optional feature in BSC so mostly SDCCH congestion shall cause pg failure.