Which protocol is used to call a Web service?
a web service protocol stack is a protocol stack that is used to define,locate implement, and make web services interact with each other .
Most will run over http/https. but the payload format can change. from things like XML for soap. to Json/Xml for Rest some more modern ones may even be using protocol buffers or similar to encode payloads.
Was there a specific webservice did you have in mind?