Almost all reputable schools in the US (is this in the US?) have agreed to an April 15th deadline for accepting/declining admissions offers, unconditionally. Rather than call the school you are waiting to hear from, you should call the program you have been admitted to and seek additional time to decide. If they refuse, you should SERIOUSLY CONSIDER whether you wish to be at a program that refuses to work with you, unethically, before your graduate studies even begin. They're essentially broadcasting to you that the only way they can get people to attend is by unethically manhandling you into deciding before most offers of admission at other universities are released.