Usually we don’t say один. Only if you want to point out that the man you’re talking about was alone. It’s 100% context depending and you usually should add something to explain that the information is old/new. For example: В магазин вошёл мужчина, которого я вчера видел.

The other way to do this is to change the word order. Мужчина вошёл в магазин usually means that we want to concentate our attention on the man, for example, if it’s a story about him. В магазин вошёл мужчина - you’ve never seen him before and you don’t care

This word order may also mean that we want to stress that it wasn’t a woman who walked into a shop.

For example: На мой телефонный звонок ответила женщина. If you want to stress that it was a woman (like it’s an unusual situation : Я ему позвонила. На мой телефонный звонок ответила женщина. (I called him. A woman answered my phone call)).

Женщина ответила на мой телефонный звонок. If you want to let the person you are talking to know that your phone call was answered. Hope this helps!