Periodically play tones when having at least one call on hold

darcs-hash:20090422145612-58103-6686f5ac6a865ed032f2e02f375b2589447ca1d4
Don't use tone signal unless other active call has audio

darcs-hash:20090422152018-58103-e5829a6b6c2814dbf829d4947f9a9ae6560e1f8e