查找一组数字中的连续范围

I have a reasonably large set of phone numbers (approximately 2 million) in a database table. These numbers have been inserted in blocks, so there are many continuous ranges of numbers, anything from 10 numbers to 10 thousand in a range. Some of these numbers are in use and are therefore marked as unavailable, the rest are available. Given a particular number I need a way to find continuous ranges of numbers, both above and below that number. The range should continue until it finds an unavailable number, or encounters the boundary of two ranges.I have a reasonably large set of phone numbers