In light of recent attempts to design context-aware mobile phones, this paper contributes by providing findings from a study of mobile phone talk in context. We argue the benefits of investigating empirically the ways in which a place is interactionally constituted as appropriate, or not, for a mobile phone conversation. Based on a study of naturally occurring mobile phone talk, we show how people handle calls in potentially difficult situations. Availability is negotiated, and it is not always agreed on whether a situation is appropriate. These findings pose challenges to the design of context-aware telephony.