Naw...my comment was meant to be tongue-in-cheek. atoi() expects a null-terminated sequence of characters. So if you want to convert it to a number with an input string of "SR125" and expect to get 25 back, you would need to use:
val = atoi(&msg[3]);
which assumes msg is a null-terminated character array. As to my comment, you could use:
val = ((mess[3] - '0') * 10) + ((mess[4] - '0');
and get the right value, but that seems to be the hard way to do it.