Starting with CME 4.0, you can enable MOH for ip phones (phone a puts phone b on hold, phone b hears music). For this to work you need to enable multicast moh. You can disable moh for certain phones by doing no multicast moh under ephone-dn.
http://www.cisco.com/en/US/products/sw/voicesw/ps4625/products_configuration_guide_chapter09186a00806a8044.html#wp1056550
I am not sure if this also disable their capability to put a pstn caller on hold. Worth giving it a shot. I have not tested this myself!!
HTH
Sankar
PS: please remember to posts!
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus