Banamex USA Customer Service Phone Number. Phone Number of Banamex USA is +1 310-203-3400 . Banamex USA is the Mexico based financial institute, is owned by Grupo Financiero Banamex S.A. de C.V., which is the second largest bank after BBVA Bancomer. The headquarters are situated at Mexico City, Mexico, instituted in the year of June 2, 1884.