Per the Cisco release notes, in version 5.1.8 of the SPA 9xx firmware, the following feature appeared:
On incoming page, do not automatically put the current calls on hold. Instead mix the incoming audio with that of the currently active calls
This functionality does not appear to exist in future firmware versions. Removal of it is not, however documented in any release notes I've been able to find. This suggests to me that lack of this is a bug. Is there any intention to address this?