ANSWER:
When one calls the Prophet salla Allahu alaihi sa sallam, If he is calling him to seek his help, this is major shirk.
But if he is calling him as a form of grief, this is permissible as when Fatima may Allah be pleased with her grieved over her father by saying O my father, you have answered Your Lord when He called you.
It is also permissible when offering salutation on him.