Hola,
quiero
una
recarga
de
cincuenta
pesos,
por
favor.
OH-lah, KYEH-roh OO-nah rreh-KAR-gah deh seen-KWEHN-tah PEH-sohs, por fah-BOR.
Hi, I'd like a fifty-peso top-up, please.
"Recarga" is the word for adding credit to a prepaid phone. You can do this at convenience stores, pharmacies, and small shops.