Are you sure that connection isn't in Mbits? Most connection companies quote numbers in bits because it makes the connection's throughput seem larger than it is. And then once you're in windows most things refer to Mbytes for transfers and storage. Though 2.5 Mbyte/s roughly translates to 20Mbit connections, which is suspicious.
This is another possible problem. They are not obligated to give you all you are paying for all the time.
If they have tried and failed to increase your speed, and no dice, they will keep taking your money for sub par service as long as you are willing to pay.
Try and find a better ISP.