A CANCEL request sent from my Asterisk UA (10.1.21.44) to FS (b-lab-1) while the call is still ringing does not work.

Why is this request resulting in a 481?

I appreciate the help - I'm still just starting to learn SIP & FS. The CANCEL request and 481 response appear as follows on my FS console:

recv 362 bytes from udp/[10.1.21.44]:5060 at 13:30:23.291616:
   ------------------------------------------------------------------------
   CANCEL sip:70904@b-pbx-lab-1.mynet.net SIP/2.0
   Via: SIP/2.0/UDP 10.1.21.44:5060;branch=z9hG4bK6f7f35ab;rport
   From: "Steve" <sip:70904@10.1.21.44>;tag=as7f6965ea
   To: <sip:70904@b-lab-1.mynet.net>
   Call-ID: 237598fd102b739a03b4a4047bf69843@10.1.21.44
   CSeq: 103 CANCEL
   User-Agent: Asterisk PBX
   Max-Forwards: 70
   Content-Length: 0

   ------------------------------------------------------------------------
send 327 bytes to udp/[10.1.21.44]:5060 at 13:30:23.292235:
   ------------------------------------------------------------------------
   SIP/2.0 481 Call/Transaction Does Not Exist
   Via: SIP/2.0/UDP 10.1.21.44:5060;branch=z9hG4bK6f7f35ab;rport=5060
   From: "Steve" <sip:70904@10.1.21.44>;tag=as7f6965ea
   To: <sip:70904@b-lab-1.mynet.net>;tag=71m745HKHKyjc
   Call-ID: 237598fd102b739a03b4a4047bf69843@10.1.21.44
   CSeq: 103 CANCEL
   Content-Length: 0

--------------------------------------

Thanks. - SW