Turn up the volume control if your telephone has this option.
If the problem is only occurring on the jack, the problem may be with that specific jack or telephone.
If you have a spare telephone that you know works, try plugging it onto the affected jack. If it works, then the problem is with the other telephone. If it does not work, then the problem is with that jack.
Is the problem occurring on all calls? If it is only happening on certain calls (i.e. when calling a cell phone) the problem may be with the other persons telephone.
Try unplugging the power fromt the phone box then plugging a known-working phone direclty into it.