When I made my calls, I usually introduced myself and asked if there was any opportunity to shadow a physical therapist for grad school applications. Although, most of my correspondence was made through email where I attached a resume as well.
You won't be seen as a burden. These offices are used to student shadows, as most PT schools now require observation hours to apply. They may have had to do observation hours themselves. Just call and email around. Once you have developed a good rapport with this PT, s/he may be able to direct you to another clinic to shadow. Good luck!
Pretty much all things observation hours related have be answered at least once if not several times in various threads here in the PT forums. I would give the forums a search and you should find all the information you need.