Arduino with A9-can anyone help me troubleshoot these errors?

Hardware Arduino GSM Shield
1

I am programming an Arduino with an A9 which has the ability to take AT commands directly. I'm using the following script, which allows me to enter commands:

    void setup()
    {
    	Serial.begin(115200);
    	Serial1.begin(115200);
      pinMode(4, OUTPUT);
      pinMode(5, OUTPUT);
      pinMode(8,OUTPUT);
      digitalWrite(5, HIGH); 
      digitalWrite(4, LOW); 
      digitalWrite(8, HIGH); 
      delay(3000);       
      digitalWrite(8, LOW);
      Serial.println("A7 Power ON!"); 
    }
    
    void loop()
    {
    	char dat;
    	if(Serial1.available()){
    		dat = Serial1.read();
    		Serial.print(dat);
    	}
    	if(Serial.available()){
    		dat = Serial.read();
    		Serial1.print(dat);  
    	}
    }

I can see that it's registered:

AT+CREG?

+CREG: 2,3,"A5FA","5CB4",0

and that it is in plaintext SMS mode:

AT+CMGF=?

+CMGF:(0,1)

OK

AT+CMGF=1

OK

but attempts to send and receive messages get a CMS ERROR 500 and 300 respectively:

AT+CMGS="0584413829"

test sms 
+CMS ERROR: 500

AT+CMGL="ALL"

+CMS ERROR: 300

Anyone have any ideas? I'm at my wits' end.