The ELS61 doesn't like authentication given without quotes: [modem3/ttyACM1/at] --> 'AT^SGAUTH=8,1,tm,t-mobile<CR>' [modem3/ttyACM1/at] <-- '<CR><LF>+CME ERROR: 4<CR><LF>' Only when user/pass strings are quoted it works: [modem6/ttyACM1/at] --> 'AT^SGAUTH=8,1,"t-mobile","tm"<CR>' [modem6/ttyACM1/at] <-- '<CR><LF>OK<CR><LF>'