While the cost of wireless service has plummeted in the last yearand mobile phones are increasingly common, many people are
now using their mobile phones to make calls across a wide region at night and on weekends, when numerous wireless companies provide unlimited airtime for a relatively small monthly fee.
这句话中划线部分的时态有没有问题啊？完成时可以用在这种时间状语前面吗 | 
