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In the SBH50 you can select which device that should be used as primary device and for calls. See this link for how to set your Nexus 7 as primary device:
That's strange. What happens if you reset the headset and only pair it with your Nexus 7?
So under Settings -> Bluetooth -> SBH50 you don't see "Phone audio" as an option?
I recommend that you try your MW1 with other devices in that case to see if this is a problem with the SBH50 or not. SBH50 supports Handsfree Profile v1.6 so as long as your tablet supports this it should work.
Sorry about the typo, mean SBH50 of course.
I'm unable to see any information about this on http://www.google.com/nexus/7/specs/. I suggest that you contact the support for the tablet to check if it supports devices using Handsfree profile v1.6.