"It is not convenient."
Translation:No es conveniente.
Whether something is convenient is more of a property than a state, don't you think? Ser is for intrinsic properties. Estar is for locations or states (happy, well, ready, resting,...). Talking on the phone is not a thing with a continuity of existence through time. What you're describing is a particular potential event (talking on the phone at this moment).
I would think a property has too much of a sense of permanence for this phrase. I would be inclined towards convenient being a short-term state.
In any case, there is sufficient doubt either way to allow either ser or estar. DL is still (at 7/15) not accepting "No está conveniente." I have reported it as an error.
This is what something 'is.' Estar is human conditions that are likely to change in the short term, or are out of the ordinary.