Was waiting for a call from Comcast to confirm an appointment but missed the call... No message.

this is a phishing call.  I had a message left stating a man by the name of Sam was calling from Comcast- that he understood I had moved and someone wanted to turn on services at my location and he wanted to help me transfer my services...  I contacted Comcast and there is no record of anyone contacting me further I was told they would not make this type of call.  they advised to not call the number.

They just called me back again.  This time I answered.

I confirm that this number does indeed belong to Comcast cable company.

He was calling to schedule the replacement of an underground cable at my house.

My advice:  You should probably answer this number. It's your cable company.

This is the number that appears on my caller ID when  the HR department of comcast calls me (I am an employee)

I got a call from this number, which I did not answer.

Calling the number back, I got a message that the number is no longer in service, but and it gave me instructions to instead dial 877-824-2288...

I did a search for that number, and found that it belongs to Comcast.

Comcast
600 W Wishkah St
Aberdeen, WA  98520-6032
877-824-2288

I think that 425-921-5800 may be a number for Comcast.

I have been having my cable serviced lately, so I think that it may have been a legitimate call of Comcast trying to contact me.

Yeah, same here.
Did not leave a message and when I called back I get a "no longer in service message"

Called me at 9 in the morning. Did not leave a message. I looked it up in at whitepages and it is from a company called Teleport Communications in Halls Lake, WA. I've never done any business with them nor have I ever heard of them. I called the number back and it says "the number you are calling is no longer in service".