It is true that a frustrating event is one which is not the fault of either party but renders performance of the terms of the contract impossible. But I reckon you are pushing your luck to claim frustration of contract here since your client is choosing to move to a place whereby he has decided he no longer the services.
What does the contract say about termination before the three year period - I cannot believe telecomms suppliers will not have thought to include such a clause to cover this possibility.