It’s a good time to be a budget cell phone company, judging by the subscriber numbers out this morning from MetroPCS. The cellular operator added 687,000 net new subscribers in the first quarter of this year, up 51 percent over last quarter’s record number. MetroPCS, which provides flat-rate service on a month-to-month basis to customers in limited markets, now has a total of 6 million subscribers vs. 4.4 million in the same period a year ago. It’s capitalizing on the financial crisis to emphasize its pre-paid cell plans, flat-rate pricing and good in-home coverage, which enables subscribers to dump their land lines. It also, however, saw churn rise from to 5 percent from 4 percent, reflecting a rise in the amount of turnover from month to month via subscribers who drop or then restart their plans.