This is, in my opinion, weakes point in 400...you have to use KT key function...if you have eg. 4 PSTN line, create 4 routes, 4 CDE and 4 KT keys. Only that way you can more-less see what is happening. But if any user use 0 (or 9) to dial out - you won't be able to see line activity (KT monitors CDE and with access code you bypassing CDE).
On 53xx work pretty stable but on 68xx expect problems with stability...
BR