Dear: AKHTAR
p = ((A^N)/N!)/ SUM ((A^X)/X!)
where:
p= propability of blocking.
A= traffic per erlang.
N= no of channel.
& the mentioned SUM is the SUM for X values from 0 up to N.
BUT notice that this formula is B(which blocked call is lost) , so there is different formula for C(which blocked call is delayed in queue system).