'numout' is a single character; fona (my interpretation of your description) wants (a pointer to) an array of characters (basically your 'numin').
So the first thing that is wrong is the use of 'numout' when you call sendSMS.
The second thing that is wrong is that you send an sms every time that you receive a character from the serial port. That does not make sense.
I suggest that you read Serial Input Basics - updated to get an idea how to process serial data and based on that modify the PC side to e.g. send something like '0123456789\n' (example 2).
Oh, and you don't need to convert stuff to integer; it's all text based.