You mentioned not having added ambience yet -- do you have some quiet room tone in? I sometimes have trouble accepting a scene I'm adding effects to without that. Also, you've probably considered this, but maybe the foley is mixed a tad hot given the sparse (lack of) background, making the gaps feel more pronounced.
Refrigerator hum, HVAC, maybe a television or radio in the next room, etc. are all things you could play with to fill out the track and add/adjust tension as the moment of the phone call approaches.
I'm a little hesitant to turn to an existing, similar scene as reference because that makes it harder to put a new spin on what I'm doing. That said, sometimes it's probably best to imitate what's come before. Looking forward to seeing the veterans here weigh in on this one.