I will leave a message for you with my secretary. It is possible that I will be out when you  telephone. 

I will leave a message for you with my secretary in case I am out when you telephone.

As long as it might happen that I am out when you telephone, I will leave a message for you with my secretary.

Unless you telephone when I am out, I will leave a message for you with my secretary.

Providing that I will leave a message for you with my secretary, I will be out when you telephone.