The ending I got definitely wasn't ideal - I might come back and try for another later on.

One thing I think would make for a small but significant improvement to this would be the option not to answer the call at the very beginning. You don't need an entire storyline there - just a basic "You reject the call and sit around all day doing nothing" would suffice - but since it literally asks "Will you pick up?" it feels a little restrictive to have only one option available.

The snappy and dialogue-heavy writing worked really well here, and the creepy build up was done very nicely! 

Effective art and interesting, breezy writing. And I agree, a bit scary.