80% of the 1000 requests are rejected –> 800 requests are rejected, in one hour.
800 TCH requests could lead to 800 * 30s of TCH occupancy, if you assume that an average call duration is 30seconds, AND if you assume there is 1 TCH request for 1 call request.
For such high congestion, you should assume that there is at least 2 requests for 1 call request (call is re-attempted by subscriber)
So in your situation : 400 * 30s = 12,000 s of occupancy is rejected, so it’s about “12,000/3600” erlang that are “lost”.