I received a call from a regular mobile number claiming to be the U+ Renewal Center...

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I picked up a call from an unknown number,
and they claimed to be from the U+ Renewal Center. They said if I renew my contract now, they'd give me 120,000 KRW in gift certificates and replace my Wi-Fi extender for free.
They even said they wouldn't charge the usual 20,000 KRW technician visit fee... they kept talking non-stop, and I almost fell for it..
When I asked why they were calling from a personal mobile number, they said people don't pick up much when calling the 1544 number, so they use a business phone instead.
They told me to call 1544-xxxx if I was suspicious,
so I just called the official U+ customer service number, 101.
U+ told me they never call from personal mobile phones and that it sounds like a call from an agency/dealer.
They also said that if I renew through the headquarters, I'd get 180,000 KRW in gift certificates, and since my current set-top box and Wi-Fi are already the latest models, there's nothing to replace... lol
I told them to contact me if they have better terms, and they said okay. They seemed really happy, asking "Are you sure you're okay with that?"—I guess they'll be calling me soon, right?