How to determine if an AT&T easement is in use and how to have it vacated or amended?

Hello,

I'm in the middle of purchasing a property in St. Louis County, MO.  There is an easement granted to Southwestern Bell in 1977 when the land was farmland.  The property was sold and later was developed into homes.  After completing a survey and pulling plat maps, it appears that a corner of the house is built in the easement.  We are trying to determine if the easement if still in use.  If so, how can we go about having the easement instrument amendment to exclude the corner of the house.  If not in use, how do go about having a document recorded that the easement has been vacated?  This is a very time sensitive matter as we will have to cancel the contract if this situation is not cleared.

Thanks in advance for your assistance.