How can i fix this?
first be more descriptive when asking questions luckily this is an easy one.
you need to adjust the normals on your model as i imagine they are facing the wrong direction. this can be done in any 3d modeling software such as blender max and maya. i assume thats the issue you meant unless you were looking to make a payphone booth.
It is also a texture problem. If you make your texture/material object double sided that will fix your problem. Just open your texture and tick two sided.
thats only really applicable if you need to see both sides of a plane as with foliage. if its basic geometry such as cube shape where only the outside will be seen then its a better idea and more performant to fix the geometry.
Thank you!