I don't know whether I read 中 as なか or ちゅう in the sentence below:
昨日寝る中で電話がありました。
I would read it as なか. You read 「plain form of a verb + 中」 as 「~~する + なか」, e.g. 「雨が降る[中]{なか}」 cf: 「勉強[中]{ちゅう}」
By the way, 昨日寝る中で電話がありました。 doesn't sound natural. You could say [就寝中]{しゅうしんちゅう}に, 寝ているときに or 寝ている[間]{あいだ}に to mean "while sleeping". Where did you see the sentence? Can you provide the context?
Since it is an individual word, it should be pronounced as なか.
じゅう(or ちゅう) is used only when 中 is an affix (attached to a noun).