This is NOT a washington mutual number. This is a scammer trying to get you to call back and give your credit card information. Do NOT do so! Wamu denies any association with these people.
I think this is some kind of telemarketing scheme. They call, don't leave a message. I've tried calling back several times only to be met with a busy signal. Lots of companies using false caller IDs these days it seems. All they have listed is the city or state of origin of the area code rather than the full disclosure they are supposed to have.