How would you write out a one sided telephone conversation?

Well I like both. I think the easiest way to write a one sided phone conversation is to write it from the point of view of a person observing. Ex:

An soft buzz broke our light conversation. Marcus swiped phone off the coffee table and I got the feeling he wasn't expecting a call. I almost laughed when I heard how gruffly he answered.

"Hello." And the change happened in seconds. Whatever the person said changed his entire demeanor. Immediately my smile vanished. Something was definitely wrong.

"Marcus! When did it happen? Is she all right?"

I would probably do it the first way you suggested. I think that sounds great the way it is.

i did this with an entire story once. heres how i did it...
"so, uuu, y r u wandering around the middle of no where? trying 2 find a place 2 sleep?! theres a town up ahead, u don't u go there? u already tryed that, hu. thats a problem, cuz the next town is 20 miles away. that's the one u came from! wait, how long have u been wandering around!? 2 months! o my..."

I write one sided convesations by writing both sides then talking only the side that would be in the story I don't no I find it easiest if I have the other persons diolouge while writing the conversation

I think the first way is better. That's what I usually do (I'm not saying it's right, but I do it anyway). Don't add every pause, just mention if he looks over at a clock or another person, if it's appropriate, or maybe if his expression changes due to what the person on the other end is saying.

I do it this way,

"Hello"..."Oh, her Marcus. What's up?"..."What? When did this happen?"..."Is she alright?"..."Alright, thanks for letting me know. I'll be right over."

That the reader knows there's something going on between the sentences, not just one long one.

You should write out something that is slightly more detailed and it cannot be too rushed, or it will look like you didn't put in enough effort. The first one is good, but I suggest the second method, but with more emphasis on how 'Marcus' was responding. The first one is better off used in quick, short calls that only last like half a minute.