In season 5, Two Loves Has Bud (1959), the Davis family moved away and the Elgar family moved in. Without explanation, the Davis family is back as the next-door neighbors, as evidenced by Bud's report that Margaret is next door at the Davis home.
Bud and Betty use the telephones in the bedroom and kitchen, respectively, to ring and connect to the telephone in the living room. The series has demonstrated that a single telephone line is used in the house; therefore, the line would have given a busy signal when Bud and Betty called from other rooms in the same house.