LAW of contract

To your first question, an offer must be effectively communicated to accept (Tinn v Hoffman), that would mean, if your voice message is not heard by the offeror, it is very possible that there would be no contract.
To your second question, if your performance is without the mind of the reward, i.e. you never knew there is a reward for looking for an lost item on returning it, the answer would be no.

2008-12-06 21:57:51 補充：
There is no contract for the reward on returning the loss item.

口頭應承也是contract,但要証明便要錄音做證據,
(對方可能有錄音).
好似銀行o甘.
事後想追回獎金比較難,因無證據,但可開心問一問,
佢係好人便會比你.