とき is a conjunction meaning “when,” so

うちにかえるときでんわしてね

and

うちにかえったときでんわしてね

both means “when you  go home, please ring me.”

The grammar difference is

うちにかえるときでんわしてね has かえる (non-past plain verb) before  とき and 

うちにかえったときでんわしてね  has かえった (past plain verb) before  とき.

When a “non-past plain” verb is used before とき, that means the action of that verb has not been completed when the action of the verb after とき happens. In short,  うちにかえるときでんわしてね means “Please ring me when you are coming home” or “Please ring me when you are on the way home.” So it is likely someone at home is asking her to ring.

On the other hand, when a “past plain” verb is used before とき, the action of that verb has to be completed when the second action happens. So うちにかえったときでんわしてね means “When you are back home, please ring me” or “After you get home, please ring me.” So it is likely someone who is not home is asking her to ring.

Other examples

日本にほんに行いくとき買かいました。
I bought it when I was going to Japan.
日本にほんに行いったとき買かいました。
I bought it when I went to Japan. (Most likely “when I was still in Japan.”)

電車でんしゃから降おりるとき、花子はなこさんに会あいました。
When I was getting off the train, I saw Hanako (on the train).
電車でんしゃから降おりたとき、花子はなこさんに会あいました。
When I got off the train, I saw Hanako (in a station).

For more beginners’ study material, please visit JLPT N5 Grammar and JLPT N5 Kanji pages.