Hi Cheech,
Only SOME interference problems can be helped with a filter in YOUR coax line. Trouble is, to tell what difference it will make BEFORE you spend the money on it.
The traditional method is to spend the money, put it in line and see what difference, IF ANY, that it makes to the problem.
Raising your antenna is the likeliest, best route to problems with a phone or answering machine. The higher the better, up to around 50 or 60 feet. Just remember that the higher it gets, the better a target it becomes for lightning, and a good ground/lightning arrestor becomes worth more than when it's near ground level.
Filters only help when unwanted 'extra' frequencies are leaking into the neighbor's gadget in the first place. With your antenna so close to the ground, the cleanest signal in the world will probably leak into nearby baby monitors, phones, stereos and such. An inline low-pass TVI filter in your coax MIGHT make no difference at all.
Raising the antenna puts it farther away from the neighbors' devices, making your (clean) signal weaker at ground level. Running the coax straight down the support pole/tower all the way to ground level tends to help. Saving money on coax by running it through the air, from where it connects to the antenna can add to this kind of trouble. The power you feed to the antenna travels on the INSIDE of the coax shield braid layer. But if you string the coax in the air below the antenna, the OUTSIDE of the coax shield becomes an antenna, and picks up your signal, out of the air. Brings it back down to ground level, where it will radiate BACK off of the coax and into nearby devices.
There are two paths that your signal can take, leaking into the A.M., through its power cord, and through the phone line plugged into it. Odds are that a "phone-line" filter installed at the rear of the answering machine itself will help more than a filter placed in your coax line.
But like I said, you never know until you try. Trying the cheapest filter first will risk less of your money, finding out what helps and what doesn't.
73