Why do some girls like to be on the phone, but at times don't say anything and just like to sit without saying anything or having a conversation? For example, this girl likes me and when the conversation starts to fade she dont want to hang up

Because it's the best feeling to be able to be with someone in comfortable silence.  Girls are awesome at talking .. And making convo ... But we don't wanna feel like we always have to. If you've met someone who is comfortable just being silent with you ... That's so awesome :)