If you are a trader who is trading for your own account, you need to have redundant systems to make sure that you have the ability to trade at all times. Even if you trade exclusively online, you should have both land line and cellular telephone access in your trading room. The reason for this is if you Internet connection goes down or your computer crashes, you can call your broker to either exit your open trades or tighten the stops on them.
In your trading room you should have a hard wired land line telephone. A cordless phone will not do. If you were to lose power at the same time that your Internet connection went down or your computer crashed or could not be used due to the power outage, your cordless phone would also stop working. A hardwired land line does not need electrical power to work so it would still operate.
Now why should you have a cellular telephone? Again as a type of insurance that is pretty inexpensive. If there was a major natural disaster and you had large open positions, you would want to be able to get out of them immediately. If you could not do it by computer or by regular telephone, you hopefully would still be able to use your cell phone to call your broker to place your trades.