Interfacing GSM Sim900a with PIC16f887

void main(void)
{
    int t,a,b;
    t = a = b = 0;
    static char Duration[3];
    static char PHnum[11];
    char Key = 'n';
    
    LCD_Begin();
    InitKeypad();
    InitGSM();
    LCD_Cmd(LCD_CLEAR);
    
    LCD_Print("Enter Phone No.");
    while(a<10){
        Key = switch_press_scan_num();
        if ((Key == '#')&&(t>0))
        {   
            PHnum[a-1]=' ';
            LCD_Goto(1, 2);
            LCD_Printr(PHnum);
            __delay_ms(200);
            a--;
        }
        else if (Key != '#')
        {
            LCD_Goto(1, 2);
            PHnum[a] = Key;
            LCD_Printr(PHnum);
            __delay_ms(200);
            a++;
        }
    };
    LCD_Cmd(LCD_CLEAR);
    while (b<10){
        phoneno[b]=PHnum[b];
        b++;
    }
    sms("Hello World");
}

void InitGSM(){
    TRISCbits.TRISC6 = 0; //Set RC6 (TX) as output
    TRISCbits.TRISC7 = 1; //Set RC7 (RX) as input
    TXSTA = 0b00100100; 
    RCSTA = 0b10010000; 
    BAUDCTL |= 0b00001000;
    SPBRG = 16; //Baud rate 115200 @ 8 MHz
    TXIF = RCIF = 0; //Disable interrupt(For reseting purpose)
}

void sms(unsigned char *msg){
    tx_str("AT\r\n");
    __delay_ms(500);
    
    tx_str("AT+CMGF=1\r\n"); //Enter GSM SMS TEXT MODE
    __delay_ms(500);
    
    tx_str("AT+CMGS="); //SEND SMS to Phone number
    tx('"');
    
    for(int l = 0; l <= 9; l++){
        tx(phoneno[l]); //Enter phone number with size 10 array
    }
    
    tx('"');
    tx_str("\r\n"); //After enter will be entering SMS
    __delay_ms(1000);
    
    while(*msg)
        tx(*msg++); //Send SMS
    tx(0x1A); //CTRL Z ASCII code to send
    __delay_ms(500);
}

void tx(char a) //Function to send one byte of data to UART
{
 while(TXIF==0) continue; //Hold the program until TX buffer is free to transmit
    TXREG=a;
}

unsigned char rx(void){
    while(PIR1bits.RCIF == 0); //Hold the program until RX buffer is free to receive
    return RCREG;
}

void tx_str(const char *s) //Function to convert string to byte
{
 while(*s)
    tx(*s++);
}