jim, Check the volume control on phone, if slider volume control, slide the control up and down quickly several time to get switch contacts to make better connection, make sure volume control is in the high range. To rule out a bad line, try a different phone in same phone jack, if the problem is there with different phone the line is faulty. If the phone is older or has been dropped a few time chances are something inside the phone has been damaged,corroded,dirty, you could dismantle phone and look for broken solder connections, wires off, etc. and try and make a repair or just replace the equipment(phone) if line/jack is found to be OK.
Good luck.
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