Sang-min: 고백하면, 문자 보내. 내 도움이 필요할지도 모르니까.
If you confess your feelings, text me. You might need my help.
Yeon-tae: 됐거든요.
Whatever.

두 번 다시 전화할 일 없으니까 댁도 나한테 전화하지 마세요.
I’ll never call you again, so please don’t call me.
됐거든요 (Whatever)

>> an idiomatic expression meaning “whatever”, “never mind”, “forget it” etc.

>> often used to reject a suggestion or rebut an opinion in a sarcastic tone

되다 – to be done
~되었다/됐다 – past tense of [되다]

Casual – 됐거든
Semi-polite – 됐거든요