C :: Write A Program That Converts Alphanumeric Phone Number Into Numeric Form

i m learning c programming and i m stuck with this program for some reason.Write a program that converts alphanumeric phone number into numeric form:

O/P:
Enter phone number: CALLAT 2255288
here is the code i have written:
/** compilers shows no error **/
and O/P I get is
Enter phone number: CALLAT
/**blank space***/

Code:

#include<stdio.h>
int main(void)
{
char ch;
printf("Enter phone number:");
ch=getchar();
}

[code]....