Access to microphone while on-hold

Hi,

I'm investigating the feasibility of writing an application that needs to do recording while a phone call is on-hold.

Assuming that there is an on-going phone call, if the call is put on-hold by the network, is it possible for an iOS app to grab access to the microphone for recording?

Or is it that even if the call is put on-hold, the access to the microphone is still held by the Phone app?

Thanks,

CJ