i experienced some cases like yours : the cell is covering a road until a certain point, after which there is no more coverage. So, subscribers are driving on the road, making calls, but the call drop after a certain point because of low UL Rx Lev & no neighbors available.
Are there neighbors in your area that should “capture” the subscribers with a low SS ? If yes, you might wan to trigger HO due to low level faster. If no, then i think it is natural to drop the call if ppl are too far away.
If the TA is very low (ppl nearby the site), you might have a antenna design issue, or TX/RX hardware issue. Setting a downtilt at 0° (or 2° if you prefer, at most) is a good idea to maximize the coverage.
I think, first of all, you should perform a drive-test in those areas and see what is really happening. Or get some Abis traces (or radio statistics) to show the distribution of timing advance compared to the UL RXLEV and DL RXLEV.