There are two good arguments against your client.
1. There was never an offer. While this may not qualify as an advetisement, I dont think it meets the conditions of being an offer. An offer must be clear and specific as to the terms, and directed at a particular party, this party being the only group who can accept. The problem is that the offer is not specific enough. Which phones, exactly are being sold? What model? The other conditions are met, however, and my argument here is a weak one.
2. The fire discharges the stores duty to perform, if indeed there was a contract. This is the doctrine of impracticability. If an unforseeable event (such as fire, flood, etc..) makes performance impracticable, the duty to perform is discharged.