Why do I get “ stray '#' in program ” error?
Use the escape backslash to get a double quote in a text. See: https://en.cppreference.com/w/cpp/language/escape

Is the *123$ between double quotes?

Serial.println("Hello");
Serial.println("I said: Hello to you");
Serial.println("I said: \"Hello\" to you");
Serial.println("AT+CUSD=1,\"*123#\"\r");
Serial.println("AT+CUSD=1,\"*123#\"");


I think the carriage return \r at the end is not needed when the println is used. The println adds a linefeed \n by the way.