While my father was abroad, he kept in touch with us by letter and phone.
Literal
Father [topic-は] overseas [at-に] is during, letters [and-や] phone [by-で] us [with-と] contact continued.
~間 marks a time span: 'during the time that X.' ~し続けた uses the compound verb ~続ける (continue doing). われわれ is a formal first-person plural pronoun — more stiff than 私たち. 接触する ('to contact') is a formal suru-verb.