Your point fails because even on hardwired lines there are charges for the safety network (911 fees they are generally called). If you want to talk fairness then what you are proposing is the landline users pay for it when you don't have to. Doesn't sound very fair to me. And taking it out of the general revenue means those who don't use it (for whatever reason) are subsidizing those that do. Again, it doesn't sound fair to me.
The point is, there are inequities all over the topic of taxes. Whether it comes from general revenue or not. And these are services that have to be paid at some point. Cell phones have caused a special headache for emergency response (911) because of their mobile nature. Getting a true fix on them can be a daunting task and increases that cost. That is why the tax as you put it is higher.
On a side note, when was the last time you saw a pay phone in a city these days? Ever try to call 911 without using your cell? Trust me, it is one of the most challenging things to have to go through.