Just to play the devil's advocate........maybe her phone was dead (battery needed charging) and as such he wouldn't have ever heard it ringing. Going one step further, if phone dead and he never heard it ringing, maybe he didn't even know for some time that she had left it behind? If they're estranged and living in the same place, it's entirely possible they each had separate sleeping areas..eg) her the main bedroom, him the guest room or sofa...and as a result she kept her personal belongings such as phone in a place he didn't go/see. Lastly, many couples who are separated remain living in the home due to costs. Right now the rental vacancy rate is very very low in Calgary, and rental prices are crazy high. If they were merely sharing the same place for this reason, it's possible that each was "doing their own thing" and no longer letting each other know their plans/when or where they were going.....so her being gone for a few days maybe didn't seem to be out of the ordinary to him. Who knows, maybe their relationship/marriage was on the rocks because she'd met someone else? Or maybe it was common for her to spend as little time at their shared place just due to the awkwardness of them being estranged so it wasn't out of the norm for her to be gone for a few days at a time without telling him where she was going/when she'd be back.
Maybe the reason she went somewhere and didn't take her phone with her was because her battery was dead? I know my iPhone no longer holds a charge for very long and half the time I go to grab it when I leave the house and it's totally dead........which isn't a big deal if I'm driving somewhere (and can charge it in my vehicle)......but if someone was picking me up to go somewhere and I couldn't charge it, I wouldn't obviously take a dead phone with me, there'd be no point.