what the fuck was she thinking was happening with the phone that could be fixed by climbing onto the roof of the stairwell? Did she think she was losing reception? lol.
Nope.
The way I see it, she hurried up to find another way down from the roof (driven by a slight hope that she can still do something to save Takishima Yuuko if given some chance to be there in time) because the regular stairwell was blocked by her angry pursuer.
Then, momentarily, a lapse in judgement: she could not force herself to end the telephone conversation or even put away the phone.
Hence she only had one hand to hold on the water pipe.
So definitely not a concious decision to end her life, but a result of not thinking straight in a panic caused by being a remote witness to her best friend's suicide.