Stuck in ISR, RCIF is not firing when UART with pic18f5420 and Sim800

RCIF is set;
read the UART to get the character;

if (CR or whatever terminates the sim message) 
{
SimMsgReceived = 1;  // tell the main program the Sim reply is complete
}

else
if ((BufferPtr < sizeof(SimBuffer)) // prevent overflow if the message is too long
{
SimBuffer[BufferPtr++] = received character;
SimBuffer[BufferPtr] = 0;  // append a null to the end of the buffer
}
RCIF = 0;
}

Then in your main program, check the variable "SimMsgReceived" and if = 1 you send the Arduino message then reset it to zero again.
You must also then use "BufferPtr = 0" to reset the character buffer back to it's first location.

Brian.

if (timerInterruptFired()) {
        INTCONbits.T0IF = 0;
        //printToArduino(""); // don't know why to add this without this blink led is not working
        ledCount++;
        delayCount++;
        if (ledCount == blinkTime) { // blinkTime=8
            ledCount = 0;
            blinkLed(); // here I just toggle the corresponding port
        }
        if (delayCount == delayTime) { //delayTime = 4
            delayCount = 0;
            delayFlag = 1;
        }
    }

and in main loop---------->>
       while(true){
                  while(!delayFlag); // stucking here 
                  printToArduino("I am a delay!!");
}

and in main loop---------->>
       while(true){
                  while(!delayFlag); // stucking here 
                  printToArduino("I am a delay!!");
}

if (delayCount == delayTime) { //delayTime = 4
            delayCount = 0;
            delayFlag = 1;
        }

use

if (delayCount == 0) { 
            delayCount = 4;  //delayTime = 4
            delayFlag = 1;
        }

char timerInterruptFired() {
    return TMR0IF & TMR0IE;
}

#include <Wire.h>

void setup() {
  Wire.begin(9);                // join i2c bus with address #8
  Wire.onReceive(receiveEvent); // register event
  Serial.begin(9600);           // start serial for output
  while (!Serial);
  Serial.println("Setup Complete!");
  
}
void loop() {       
  delay(100);
}
void receiveEvent(int howMany) {
  char c;
 
  while (Wire.available() > 0) { // loop through all but the last
     c = Wire.read(); // receive byte as a character
     if(c=='\n')
     Serial.println();
      else
    Serial.print(c);
  }
  
}

void Timer0Setup() {

    /*
     * FCPU=8MHz/4= 2MHz
     * TimePeriod = 1/2MHZ = 0.5 us.
     * PrescalerPeriod = TimePeriod x 256 = 128us;
     * OverflowPeriod = PrescalerPeriod x 2^8 = 32768us ;
     * So we need 1/OverflowPeriod = 30.51 ~ 31 - Overflows to count for 1 sec 
     * 
     * TMRO = 256 - (Delay time *fosc/ 4*prescaler value)  
     * 
     * taking Delay time = 10ms then 
     * 
     * TMR0 = 248
     */
    T0CON = 0xD7;
    TMR0IE = 1;
    RCONbits.IPEN = 1; // ENABLE interrupt priority
    TMR0 = 248;

}

void main() {
    OSCCON = 0x75; // INTOSC frequency 2MHz
    TRISBbits.RB7 = 0;
    UART_Init(9600);
    I2C_Master_Init(9600);
    InterruptSetup();
    Timer0Setup();
    while (1) {
        while (!delayFlag); // still stucking here 
        printToArduino("I am a delay!!");
    }
}


void interrupt interruptRoutine() // High priority interrupt
{
    if (timerInterruptFired()) {
        INTCONbits.T0IF = 0;
        TMR0 = 249;
        ledCount++;
        delayCount--;
        if (ledCount == blinkTime) { // blinkTime = 400; 
            ledCount = 0;
            blinkLed();
        }
        if (delayCount == 0) {
            delayCount = 300; //delayTime = 400
            delayFlag = 1;
        }
    }
}