If you want = если вы хотите
If you need = если вам надо/нужно
Normally, people who call the fire brigade do it because they need to, not because they want to :-)
One minor correction, you should use the determinate article THE, not the indeterminate article A. Also, normally people would call the Fire Department (the "brigade" would be the small team of firemen that get dispatched, but you wouldn't be able to call them directly!): "If you need to call THE fire department..."