As stated in the title, I would like to know when to use either. I read the documentation for REQUEST and BEGIN_REQUEST

I understand, one of them is for Single HTTP call and the other establishes HTTP connection. But when we need to use established HTTP connection, and when we need single call. I used BEGIN_REQUEST to POST data from Oracle to MongoDB in the past, and GET data as well. There was a URL, and another header parameter contains the content that is being sent or received. But then, I tried to use it to send SMS where the content is part of the URL, although I received the SMS, there was some error from UTL_HTTP package TNS-12545:  Connect Failed Because Target Host or Object Does Not Exist tips.

Can BEGIN_REQUEST END_REQUEST always replace REQUEST? How about the opposite?