The reason is because the main door is considered the mouth of Qi and the mouth of your house. The door is also considered the central location where external Qi comes into the house. A narrow telephone pole directly facing the main entrance door is acting like a knife or blade slicing through your main entrance door. In term of FengShui it is very important to protect our main door from this kind of attack.
There are a few methods that can be used to rectify the problem. Some cost less and some cost higher.
1. The most effective way is of course shift your main door to another location with the same or different facing.
2. If there are steps to your main door, you can consider semi circular steps at the front entrance.
3. One other effective way is to have a semi-circular carpet radiating outwards from the house.
Note : FengShui practioners classify a tall thin structure as Wood element while others classify the telephone pole as metal simply because the pole is made of metal.
4. You can place a Pa Kua directly above your main entrance,
5. Changing to a convex door. This might not be easily available unless it is specially ordered. Tilting the door to face one of your auspicious directions. (Caution: You would need to consult a Feng Shui expert for this. He will also need your BaZi Four Pillars of Destiny). The advantage here is if one's main door is facing an inauspicious direction, one can tilt the door to face an auspicious direction. Here, you are killing two birds with a stone because it will also solve the problem of the poison arrow caused by the telephone pole.
6. One other method is to place a pot of a medium sized plant on the path of the telephone pole to soften and to neutralize some of its effects.
The above are some of the common methods used.