Unable to receive response from SIM900
A number of mistakes here:


Waiting for a byte to come in inside the RX interrupt; the fact that you're there pretty much tells you a byte has arrived.
Reading 20 bytes when all you have is notification that one byte has arrived.
Assuming that a response from the module will be exactly 20 bytes. You can see from your terminal that 'OK' + a bunch of CRLFs is returned when you send 'AT' so why read 20 bytes? 
You appear to be using the same UART lines for talking to the SIM900 and your PC. Way to cause confusion.
You are sending data within the ISR to the PC. ISRs are meant to run quickly and return. 


You need to use carefully chosen timeouts to know when the module is done talking to you. Your handler should look like this:

void rx_handler(void) {
  Rx[k++] = getcUSART(); //read a byte from UART
  lastRead = 0;
  PIR1bits.RCIF = 0; //Clear the interrupt flag
}


I assume k is a file scope variable and is declared volatile. Since you are using a PIC and so dont have the Arduino millis() or ARM SysTicks readily available, you can either implement something similar with a Timer peripheral or use delays as a short-term work-around.

Declare these:

  #define TIMEOUT 100
  volatile static unsigned int lastRead;


Your main() should look like this:

  Uart_Send("AT\r");
  while (lastRead &lt; TIMEOUT){
    __delay_ms(1);
    lastRead++;
  }
  if (strstr(Rx, "OK") != NULL){
    while (1){
      LATBbits.RB0 = 0; // blink quickly
      __delay_ms(500);
      LATBbits.RB0 = 1;
      __delay_ms(500);
    }
  }
  else {
    while (1) {
      LATBbits.RB0 = 0; // blink slowly
      __delay_ms(1500);
      LATBbits.RB0 = 1;
      __delay_ms(1500);
    }
  }


Set up RB0 as an output pin and connect an LED.
I have used LED blink rates to indicate success or failure of your command. If you want to read the actual reply, either get an LCD or set up a Software UART port for talking to the PC; IIRC, the peripheral libraries include a Soft_Uart library. 

Understand that this code is only to test if you can talk to the module. For any interesting uses, you'll need to implement a ring/circular buffer and Software UART, if you want to talk to your PC as well. I suggest you find an existing SIM900 library and try porting the relevant functions, used for sending commands and parsing responses, to C18 C.