I have a sprint PCS phone. I was wondering if there was a way I could
use my cell phone and call someone else's cell phone (unknown service
provider) and leave a voicemail without calling them directly (having
their phone ring)?

Have they left a message on your phone? If so, you may have that
option in voicemail menus provided.

There is no way to do this.....sorry

You might find some of this information useful:
http://www.bridog.net/cellular/voicemail.txt

I never had this done to me, I just heard about someone doing it
before, and I wanted to know how.

There is a way to do this.  The only catch is the number you call HAS
to be a Sprint number and also HAS to be the same home area.  For
example a San Antonio Sprint number area code is 210.  You have to
call another Sprint customer with the 210 area and with a prefix that
is used for San Antonio numbers only.  I worked at Sprint for 3 years.
 Let me know if this is what you want, then Ill answer how to do it.

How do I mark this question as answered?

The facility will be there in that cell phone.. the caller cant
directly give a voice mail.. If the phone is put on voice mail then
only u can give a voice mail.. catch me on just4u_rash@yahoo.co.in