I think I know the place you're talking about. Is it near the restaurant, and the gift shop with the Toshiba PBX? I was there around Christmas of 2011, and there was still at least one fortress out there. It was cold and this particular phone was near a not particularly good smelling bathroom
Don't know what kind of PBX they have, but this was the fortress upstairs in the restaurant's waiting room, on the other side of the wall to the right of the front desk as you walk in. If the old GTE/Automatic fortress is still downstairs by the bathrooms, then it's the same type as in the restaurant. I think there may have even been another in the waiting room to the left of the doorway as you walk out to leave.
And yeah, though the bathrooms there used to be way worse (hopefully you didn't have to see them for most of the 90s), they still leave a lot to be desired in the way of cleanliness. This may explain why the espresso stand, when it's operating, now does so outside near the snack bar area on the building's east side!
There's something about that particular telco and shitty payphones.
Well I'll tell you, it was once United Telephone turf. That company was once EVERYWHERE in the gorge, from probably Sam Hill's place down to Troutdale. If you can imagine all the horror stories you ever may have read about GTE and U.S. West combined then multiply them by a factor of maybe several hundred thousand, then you'd have a general idea of how life on UnTel was like. (It's really no surprise, then, that in the 80s they were bought out by SPRINT, which subsequently merged with GTE because they were all apparently a match made in heaven.) Shitty telcos and shitty payfones go hand in hand, and I wouldn't be surprised if they were left over from that era.