How do you fix a telephone line crossover problem?

We are a company with multiple phone lines. Ever since the beginning of 2007, after some strong Santa Ana winds in Southern California, we have been experiencing a crossover problem with a few lines on our phone. When we are on the phone, we occasionally hear an intercom system from one particular company. We... show more We are a company with multiple phone lines. Ever since the beginning of 2007, after some strong Santa Ana winds in Southern California, we have been experiencing a crossover problem with a few lines on our phone.
When we are on the phone, we occasionally hear an intercom system from one particular company. We never get the name of the company. We just hear people broadcasting over the intercom calling for so and so person to report to so and so department.
We have called Verizon telephone company to come and check, they found nothing. We even tried the Edison power company, they were no help. We have called several of our neighboring buildings asking them if they have an intercom system that calls out those specific requests, they all said no.
The problem is getting louder and more frequent. We can hardly even hear our own phone conversations because of the cross over.
Does ANYONE have any idea???? Help!
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