I agree with your partly.
Failure attemps might be coursed by some reasons, such as lack of traffic resources, lack of signal resources, radio failure, element trouble, etc. We only take lack of resources in to consideration in blocking rate but others.
Erlang B table working well under 5 conditions. A GSM network is not complied all these conditions in cell level.
I do not take handover as the most serious problem.
But if each of your cell only has a few channels, you need correction for your sum up when you figure out your A-interface and capacity of MSC.
On the other hand, radio interference will ‘block’ some of your channels even not in use.
Hope this information can help you more or less.