You can do that by means of "indirect variable reference". Use the notation ${!variable} introduced in Bash version 2:
Code:
#!/bin/bash
func_count_fmfm () {
echo "Starting FMFM counts for ${category}"
eval "fmfm_umbillingnumber_count_${!category}=0"
echo $fmfm_umbillingnumber_count_${!category}
}
category="biz"
func_count_fmfm
category="res"
func_count_fmfm
The eval is mandatory to let the shell do the correct substitution.