Después de mucho buscar en Google y combinar varias respuestas, lo hice funcionar.
Aquí está la secuencia de comandos AT. Puede probar esto de principio a fin, solo tendrá que confirmar el APN del operador que está utilizando.
// My comments are here
Command to send is here Expected responses are here
// See if the SIM900 is ready
AT OK
// SIM card inserted and unlocked?
AT+CPIN? +CPIN: READY
OK
// Is the SIM card registered?
AT+CREG? +CREG: 0,1
OK
// Is GPRS attached?
AT+CGATT? +CGATT: 1
OK
// Check signal strength - should be 9 or higher
AT+CSQ +CSQ: 14,0
OK
// Set connection type to GPRS
AT+SAPBR=3,1,"Contype","GPRS" OK
// Set the APN - this will depend on your network/service provider
AT+SAPBR=3,1,"APN","wholesale" OK
// Enable GPRS - this will take a moment or two
AT+SAPBR=1,1 OK
// Check to see if connection is correct and get your IP address
// (I hid mine here, but you'll get a real IP back)
AT+SAPBR=2,1 +SAPBR: 1,3,"0.0.0.0"
OK
// Enable HTTP mode
AT+HTTPINIT OK
// Set HTTP profile identifier
AT+HTTPPARA="CID",1 OK
// Put in the URL of the PHP webpage where you will post -
// the URL below is a test server so you can use it in testing
AT+HTTPPARA="URL","http://posttestserver.com/post.php"
OK
// Tell the SIM900 how much data you'll send (18 bytes here)
// and how long to wait for a time-out (10,000 ms here)
AT+HTTPDATA=18,10000 DOWNLOAD
// Key in the data you want to send after you get the "DOWNLOAD" prompt.
vdc=12&adc=3&rel=8 OK
// Post the data - wait a second for the response
AT+HTTPACTION=1 OK
+HTTPACTION:1,200,142
// Read the response - www.posttestserver.com will give you a
// URL where you can confirm that it's working
AT+HTTPREAD +HTTPREAD:142
"Successfully dumped 0 post variables. View it at
http://www.posttestserver.com/data/2016/04/28/14.19.041655610622
Post body was 18 chars long."
OK
// Close the HTTP connection
AT+HTTPTERM OK
// Disconnect the GPRS
AT+SAPBR=0,1 OK
Editar para agregar: Aquí están las fuentes que solía juntar todo esto, junto con la ayuda de algunas personas que pregunté en mi laboratorio.