Kare wa sensei desu. (It's an explanation ro someone) we usually use this sentence when someone(speaker) say about his(in this case) occupation. we use in daily conversation. Kare wa sensei to iu koto desu (It's an explanation but more emphatic) if we use this sentence, when someone say about his(in this case) occupation. but someone(speaker) want a just little to emphasize about occupation in conversation. Some one told me that "Watashi wa eki ni tsuita toki, o-tou-san ni denwa shimasu" is wrong, "Watashi wa eki ni tsuita ra, o-tou-san ni denwa shimasu" is better why because [denwa shimasu]=future sentence [eki ni tsuita ra]=future sentence [eki ni tsuita toki]=tense is not sure [toki]=we can use future sentence and past sentence "eki ni tsuitara, o-tou-san ni denwa shimasu".means↓ "eki ni tsuitara,(after that) o-tou-san ni denwa shimasu". Watashi wa NIhon ni itta toki, o-miyage wo kaimasu. is wrong, why because [o-miyage wo kaimasu]=future sentence you can say Watashi wa NIhon ni itta toki, o-miyage wo kaimasita. why because [o-miyage wo kaimasita]=past sentence in this case [toki] explain past. Watashi wa Nihon ni ittara, o-miyage wo kaimasu. [ittara]explain future. I understand your feeling. Actually my feeling is quite opposite of English.

Oops...see above.

Watashi wa NIhon ni itta toki = When I WENT to Japan o-miyage wo kaimasu. = present tense, so not good. Hmmmm..... Nihon ni itte, omiyage wo kaimasita. Nihon ni ittara (when I GO go to Japan), omiyaga wo kaimasu. It's the same problem with "denwa shimasu". Kare wa sensei desu. = neutral sentence and doesn't explain anything. It can only answer the neutral question "Kare wa nani wo shimasu ka?" These neutral sentences are RARE in daily Japanese. Kare wa sensei to iu koto desu = is an explanation, but I can't give the reason because I don't know what precedes. This kind of sentence is more common in daily Japanese.