Actually, you can do this. But you would be charged another new service fee. It's much cheaper to run another line or go wireless.
If you don't feel like laying networking cable or going wireless, you can look into power-line networking. It's usually about $40 to $50 for a kit. It runs about 14MBps. So if you have lead walls, no phone, cable, or networking jacks, and need an alternative- there it is

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