Nexus 7 cannot acts as Handsfree
Bluetooth has a master/slave setup, in which the other device is a master (for example, a phone) and the other a slave (a headset). Most phones support the hands-free profile only as a master, which means that the phone can connect to slave devices and stream its audio to the device, not the other way around. Apparently Samsung has included support for HFP slave on it's Player models, which is reasonable since they don't have phone functionality builtin.

Therefore, it's not a question of settings, it's a question of support.