I see the potential here. It's just really underdeveloped right now. Give us some action in the story and character development. Specific things I noticed....
I'd agree with everything Steven said.
Every slugline should have a TIME (DAY, NIGHT, MORNING, CONTINUOUS)
Jeff's phone conversations are uninteresting and long winded. Either give us some information, shorten them, or break them up using actions. Seeing a long block of text and having it not add anything to the story is a complete turn off.
Are all these Woman/Man characters the same person or different people? You shouldn't have characters (even minor characters) have the same "name". I'd suggest using Woman #1 and Woman #2 or Woman and Young Lady. Pretty much something to let us know they aren't all the same person. I was confused in the beginning why we were paying attention to Man and Woman who really have nothing (that I can see) to do with the story.
We should have Character descriptions for Jeff and Voice. We pretty much know nothing about the two main characters. As for Voice, even though it is just his voice, a description of what this person sounds like would be helpful. Is it a male voice? Female voice? Does the person sound very methodical like Kiefer Sutherland in "Phone Booth" or like Mark Hamill's Joker? Describing the personality of the voice is just as important as describing a character we can see.
Dialogue is of the "on the nose" variety. The characters say exactly what they are thinking which doesn't really hold interest. Spice it up.
Story in general doesn't really move. Give Jeff something to do other than walk around. Maybe he tries to pry the phone away with a stick. Maybe he thinks splashing water on it will make the phone "less sticky" so he can remove it. Have the other people in the park make suggestions or attempt to help him. Having two spectators pretty much say "you're lying" "You're screwed" doesn't hold interest.
Give context to why the phone lets go when Jeff says "I love you." I get what you are trying to get across but there's not substance to it. By that I mean, give us a hint at why Jeff doesn't currently feel like he loves himself. Maybe the phone conversations he has earlier could be a conversation with a girlfriend breaking up with him. That would damages one's self-perspective.
Word choice - Since Jeff WALKS around the park pretty much the entire story, turn that simple act into something. Have him WALKING, but then, when he panic starts to set in, he can SCURRY or DASH. Right now I just picture Jeff calmly walking around the park with the phone to his ear. I'm not sure i'd react that way in this situation.
Hope this helps.