Can "cheer up" be used only when someone is sad
From what I understand, to cheer up means to feel less miserable or less sad.

e.g. He was feeling blue because he had a rough day at work, but an unexpected phone call from her cheered him up.

But can we use "cheer up" when the subject wasn't feeling miserable in the first place? Say, in the above example, if he hadn't had a rough day and was not feeling blue, could we still have used "cheer up"? If not, what can we use instead?

He got home after a typical day at work. An unexpected phone call from her... made him happier? Filled him with joy? brightened up his day?

Question

Can "cheer up" be used only when someone is sad
From what I understand, to cheer up means to feel less miserable or less sad.

e.g. He was feeling blue because he had a rough day at work, but an unexpected phone call from her cheered him up.

But can we use "cheer up" when the subject wasn't feeling miserable in the first place? Say, in the above example, if he hadn't had a rough day and was not feeling blue, could we still have used "cheer up"? If not, what can we use instead?

He got home after a typical day at work. An unexpected phone call from her... made him happier? Filled him with joy? brightened up his day?

Answer

'Brightened up his day' is a nice expression, because it suggests that he was having a neutral kind of day - neither sad nor happy - and call brightened it up.

Answer

Cheer up is only really used when someone was sad etc before hand. But it can be used in a general way by some people, The call from his freind cheered him up. It may not have indicated he was unhappy in the previous sentences but the call did cheer him. But in majority of cases it is used to indicating someone cheering up after being sad/ill.
You could say something like the following if they were not sad/ill before.
He felt so happy after her unexpected call.
Because she called, it gave him a big boost to his ego 
Her call brightened up his day.
His day seemed brighter after her call.
After her call, he had a smile on his face for the rest of the day.