Is there a way I can prevent the blackberry from going into sleep mode and disconnecting calls when I holster it in my new Seido Swivel clip for the 8700c? Can this function be turned off on the blackberry?

No. But you can try to remove the magnet from the Seidio holster and that will do the trick. Or you can set the holstered settings to exactly match those of the non-holstered settings, thereby making the only difference the screen turning off.

Why not go into the phone app and into General Options and switch "Auto End Calls" to "Never" (instead of "Into Holster")? It won't end calls when placed into the holster unless configured to do so.