"yo-boh-say-yoh" (corrections on how to spell or write it properly super welcome).
여보세요

Here's a few short phrases that someone at conversational-level Korean could probably make good use of:
1. When you're trying to remember something, and would say something like "what was it again?" in English:
뭐였더라? / 뭐였지? (they mean the same thing)
e.g.
아... 그 남자... 걔 이름이
뭐였더라? 생각이 안나.... (Ah.. that guy... what was his name again? I can't remember it...)
2. When responding to a question and not fully agreeing with what's been asked, it's common to begin your sentence with '글쎄....', which means something like "hmm...", "well...", or "let me see...."
e.g.
A: 한국이 꼭 이긴다던데 너도 그럴거야? (I heard Korea will definitely win - do you think so too?)
B:
글쎄... 정말로 그럴 수 있을까? (Hmm... is that really possible?)
3. Another which I get some mileage out of is '뭐랄까', which means something along the lines of "how should I say" or "if you like" - when you're trying to think of the correct word or phrase to use in a certain situation.
e.g. 안경을 왜 집에 두고 오느냐면 난 그냥...
뭐랄까... 나이가 들면서 건망증이 심해지고 있는걸? (As to why I left my glasses at home, I'm just... how should I say... getting forgetful in my old age?)
disclaimer there might be some mistakes in my Korean sentences, but the general usage of the words should be correct!