ext range feature = subscriber can be located from TA=64 to TA=128 or more ? Instead of from TA=0 to TA=64 ?
Are you sure that, in an urban environment, your cell can cover further than TA=64 (35km) ? That’s very surprising..!
On the system i’m working on, the extended cell cannot handle calls which are located between ta = 0 to 64. The extended cell can handle only calls with TA > 64. It’s like a separate cell.
Maybe your problem is that the highway is located at a TA < 64 ? So the "inner" cell will handle all the calls. In this case the only solution is to increase your capacity. Add a microcell to cover the near-by city area, or add TRX on the current cell. In GSM specs, as far as i know, there is absolutely no way to prevent near-by subscribers to access the cell, while allowing far-away subscribers.