Complete AT Command reference for the Multimodem Line is http://www.multitech.com/en_US/documents/collateral/manuals/s000293j.pdf.
1. First connect to the modem from the terminal with
> screen /dev/ttyUSB0 115200,ctsrts
2. Run the AT Command for Unstructured Supplementary Service Data +CUSD
> AT+CUSD=1,*556# [enter]
Here I’m using the check balance command MTN Nigeria. You can use this to reload credit as well with the *555*CODE# command.