[henry]

I think your example is far from perfect. Sure you may be in the mood to do work, but if you don't have a phone available then you don't want to be presented with tasks that require a phone.

Having said that there are definitely situations where an action could have more than one context. As an example I have an action "Ask parents about labour weekend".
I can email them or phone them or ask them in person. Each one is equally valid.
But does it really matter? Sure if I put it in the Email context and I'm making phone calls I'll miss an opportunity to call them, but so what?
I'm feeling good about ticking actions off my Phone context and I'll get to my Email context later in the day or the next. I will get to my parents then. Does it really matter?
If it's critical that I call them 'now' then it will have a due date or be in my calendar and it won't matter what context it's in.