Understanding rural women's preferences for telephone call engagement with primary health care providers in Nigeria: a discrete choice experiment oleh Joanna Schellenberg, Zelee Hill, Tanya Marchant, Nasir Umar, Abdulrahman Shuaibu, Maryam Ibrahim, Nuraddeen Umar Sambo, Abdulkarim M Aliyu, Kallah Kirpu Kulani, Muhammad U Abdullahi, Ahmed Usman, Hafsat Mohammed, Hajara Adamu, Adamu Mohammed, Adama Abdulhamid, Zainab Muhammed, Afodiya Alfayo