Phone scam

I received a call and voice message from [Edited: "Personal Information"] to call [Edited: "Personal Information"].  They said I can get a 50% discount on my monthly service bill. I called the number and it seems like a legitimate Xfinity number.  I wait a long time and a person answered.  He said Target is sponsoring this discount and asked me to sign a three year contract at $15/month but I need to prepaid two years in advance which comes to $300.  They asked me to go purchase a Target gift card with $360 loaded into the card and call them back on [Edited: "Personal Information"] before 10pm tomorrow!  When I call this number, I am to give them the authorization code “XFI618”.  They also asked me to sign into my Xfinity app and check that they gave me a $35 credit already.  I checked and indeed there is a $35 credit in my account!  They also pressured me to take care of this within 100 minutes and I will get another $100 Visa gift card!  This prompt me to be suspicious of this call!  I would like to know how they can get hold of my Xfinity account and triggered a $35 payment!  Can you explain to me what is going on and confirm this is indeed a scam!