Hi there,
What kind of error are you getting?
I thought crontab command doesn't output anything upon success (I may be wrong). In that case, you may not see anything even if it was successful, because system() returns the last line of system output. Try cron -l to see what jobs you have in crontab.
Also, there may be some issues with permissions - something to do with /etc/cron.deny or /etc/cron.allow file, but I'm not too sure of that.
By the way, by importing cron.tmp, I guess you are replacing the existing cron jobs with whatever you have in the file - did you make sure to include all the default/existing jobs, etc, set by your admin or control panel or whatever it may be?
Sorry I can't be a help really, but my 33 cents there.