There are probably many subscribers crossing the borders during the day (for work ?). Their MS will attach to your network (with a Location Update).
You can check this by executing the report called :
Mono Object Evolution / Alc_Mono_SDCCH_traffic_model
It will show you the amount of loc. upd. in this cell.
You can’t really do anything if people are coming in your country. Just be sure that you have enough sDCCH timeslots in this cell to carry all the required SDCCH traffic. Avoid SDCCH Congestion.
If a MS is not attached, it cannot make a call 🙁