Hi Andrew, Yes they do prefer a land line, but in Singapore mobile phone connection should be very good. Make sure you are in area, where signals are good. They wouldn't know if the line is land or mobile as long as you both can understand each other. I had the same problem, and just gave her mobile phone, and we talked for 1 hour without any issues.
The only thing you need to worry about, that calls automatically get disconnected at 59:59, so that;s why ideal would be land line. Then its your luck if the interview is over an hour, you phone will be disconnected.
Good Luck and wish you all the best.