Problem with SIM900 + Arduino UNO

Hi there,
It turns out that I am trying to connect the SIM900 module, and when I go to test that everything works correctly by sending it a simple AT command ... it returns numbers instead of OK.
I haven't seen this happening to more people so I don't know how to fix it.
For any command you enter it returns numbers. I have tried to enter the command with the pin but it does not find the network (according to the blinking of the led). I do not know if it could be the sim that I am using, I use the same one as on my mobile, and before I have tried to connect only to 2G networks to see if there was a signal, and yes there is. This is a Yoigo sim, a Spanish company. I do not understand why it can be, I attach the images that I have taken.

If I do not solve it I will try to change the sim, or return it and ask for another module. Thanks

Which characters are from SIM900.read and which are from Serial.read?

Looking up what is being returned at asciitable.com
10 21 20 12 81 58 25 41 34 25 4 - seems to be short a character?
LF NAK DC4 FF Q : EM ) " EM EOT

If it leading 0 is received and just not being printed,
01 02 12 01 28 15 82 54 13 42 54
SOH STX FF SOH FS SI R 6 CR * 6
Which doesn't make much sense either.

I tried this:


And now i'm getting ¿¿ , i shouldn't be receving that answer, could it be the module, or maybe this sim card does not work with it idk :frowning:

Update on my situation:
I ended up returning the SIM900 and purchasing a SIM800L. Now working perfectly, same code, same sim... I didn't solve the problem but i managed to escape from it.
Thanks for reading and responding :slight_smile:

code:

#include <SoftwareSerial.h>

SoftwareSerial SIM(10, 11);
char data;

void setup()
{
  Serial.begin(9600);
  SIM.begin(9600);
  Serial.println("READY");
  delay(3000);
}

void loop()
{
  if (SIM.available() > 0) {
    data = SIM.read();
    Serial.print(data);
  }
  if (Serial.available() > 0) {
    data = Serial.read();
    SIM.print(data);
  }
}

circuit:


some tests :

imagen_2021-03-11_122031.png

Moderator:
Cross post.
Spanish thread has been closed.

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