A mobile phone is going to roam only if it has lost its home coverage, or if the neighbor cell from the other country provides a better coverage. The second possibility would be an inter PLMN reselection, and I’m not sure whether it’s possible or not 🙂 Anyway, we can try to avoid reselection.
So why are your subscribers loosing your coverage ?
1/ BTS is down
2/ Wrong cell selection / reselection settings (check RXLEV ACCESS MIN value, should be around -110dBm at country boarder, increase the CELL RESELECT HYSTERESIS up to 10 or more dB)
Also, you might want to increase the Cell Reselect Offset. Perhaps the other operator have increased its CRO so much that their cells pull your MS’s out of home PLMN. As I said before, that would be an interPLMN reselection, and I don’t know if it exists or not…
Or perhaps I did, but then I forgot 🙂
Have you perform a drivetest in this area ?
I assume you declared the neighbor cell (from other country) as a proper neighbor ? If yes, during a drive test, you can see why a MS would leave your home coverage. Contact the customers that complain, go to their house, and see why it’s happening to them.