Going to divide this into two sections.
Clear part.
Question 1.
No, with your average cell phone, you can not reach 30 miles. The reason make a build out with many towers, is to make a spiderweb coverage. The phones are designed and made to use the spider web system. The spider web system will cover about 30 miles around target city.
It would be pointless to make a usable cellphone that would have that much range. It would be literally be like using a .50 caliber gun to go duck hunting. It would just be numbing to work it out in a high tower area.
Question 2
Femtocell - Wikipedia, the free encyclopedia
There are also repeaters (sprints overdrive, and clear spot) and other devices. The range is what every the base station is set to usually 5-10 miles. But can be up to 30.
There will come out a wimax broadband router. That will use wimax connection to repeat both wimax/wifi signal. These will be used for cars and mobile objects.
I have said it before, I will say it again, and make people mad. Wimax is an extension of wifi. It is made to extend the network from lan (local area network) to Man (metropolitan area network)
Not clear part
wimax does not use voice. If it does it is just voip which is data.
Ok we will just talk theoretical here. Because it does really depend on way too many factors.
If all limitations where removed. Yes you should be able to do that. In real world the limit is set to about 4-5 miles, for many reasons, because you are dealing with 90 or so base stations in a 30 mile area. Just like modern phones. But you could do it for 30 miles. And I have seen it work with 30 miles. In nomadic mode.
Your gsm/cdma phone can reach 25 miles. So it is not out of the question.
The problem with that in no real world area would you want to do that.
But there is no clear answer to your question.
There are too many issues. Too many questions. I have seen it done, but only on limited bases.