Lost dial tone

is your unit is new?or your battery is new?if not,then it is probably your battery is weak.you cant say that a battery is GOOD if it still accepting a charge when you are going to charge...let me say that even if a power is stored on it ,you still can't say that it is still good.Becoz when a battery is weak,its current or ampere will become lower than the given specification when it was new.Thus,when a current is low,a batery can no longer supply the exact needs of the unit during the time of conversation,and thats the reason why it will shut down in just almost a minute of conversation even you know that it was fully charged .