These are all examples of acceptable sentences:

– Harry, travelling to the tutoring session, received a call.

– Harry received a call, travelling to the tutoring session.

– Travelling to the tutoring session, Harry received a call.

– Sam, however, hates math.

NO comma before that:

– We all know that Ramaz is a great school.

– My dog that is sitting in the yard is a golden retriever.

– He found out that the public preferred live magic acts to filmed versions.

DO use a comma before which:

– My dog, which runs to catch the yellow ball, loves to eat.