ANSWER:
This is from the prescribed adhkar of Salah, which have to be recited as we had been taught by the Prophet salla alaihi wasallam.
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.