Do the following sentences differ in meaning? I understand that conditional たら adds the meaning of simultaneous action, but does the meaning of the sentence really change without たら?
寝ようとした、電話が掛かってきた。I was about to go to bed, and the phone rang.
寝ようとしたら、電話が掛かってきた。When I was about to go to bed, and the phone rang.