I emailed ecare2@cc.sprintpcs.com after getting a couple of calls from this number.  This is the email response I got from them:

"Thank you for emailing Sprint. I understand your concern regarding the promotional calls.
I apologize for the inconvenience causedregarding the promotional calls.I have blocked the feature of promotional calls on your account. Now,you would not receive any promotional calls from Sprint.
Sprint appreciates your business and I am only an email away if you have any other concerns. "

This is what I wrote:
*I'm a Sprint customer, my cell # is ---------.
I want to stop getting any type of calls from you.  I'm not interested in 'special offers'.  In fact, now that my contract has expired, getting another annoying call from you will only make CANCEL my plan. Not only that, but since I have in the past requested to be removed from your calling list, I will find a way to file a formal complaint.*

So there IS a way to stop the calls without having to waste your time calling them (we all know how long that takes!)

Same history about Sprint Customer Service ... They want to verify my name and address. I asked him what number you dialed, and he never gave me a right answer. I've received these type of calls since 3 weeks ago, and it is worse in one of the calls they mentioned about the credit cards that I have ...
Bottom line, they want to confirm all the info that they have about you to do SCAM.

I just got a hang-up from that number and Googled it.  I found this message board.  If it's actually some clown phishing, I hope somebody finds out where they are and pays them a visit.

They call and hung up. Upon calling back, the line is busy or out of service. The Goverment needs to pass a law makes all 1-800 or 1-888 listed for the public.

Called me at 3:52 PM Tuesday and said he was from Sprint.  He called me by name and praised me for being a long standing Sprint customer and had special offers.  For verification he asked for my PIN number.  I said I didn't remember it.  He thanked me again for being a Sprint customer and hung up.

Here are two things that made the "Red Danger Sign" go off in my head:

1. I don't give any information to anybody who calls unexpectedly.  Somebody could be phishing

2. I gave permission for Sprint to send me offers by email or snail mail, not on my cell phone.

I got a call today and Monday at the same time 4:08pm...I answered and they hanged up. I called back it was busy.

Got several calls from this number.  No one there.  Called the number back and got a message that the call could not be completed as dialed

I GOT A CALL FROM THIS NUMBER AND THEY JUST HUNG UP .

I also got a call from them & they hung up, I called back & got a note that the code was incorrect.

i got a call from this number and then they just hanged up.

this # calls and listens to my voice then hangs up.

I hate Sprint that is why I am leaving them, they are just probably messing with everyone. B***h Sprint.

this # calls and listens to my voice then hangs up.

FOR TH PAST 3 DYS I HAVE BEEN GETTING CALLS FROM 888-680-1668, NO MESSAGE IS LEFT AND WHEN I CALL THE CIRCUITS ARE BUSY IT SAYS TO CALL BACK LATER....

IF ANYONE DOES FIND OUT THE TRUE IDENTITY OF THIS NUMBER PLEASE POST. THANK-YOU

I get the same call/hangup from them.  Sometimes several times in a day.  If I could locate the source I'd most definitely pay them a lil visit-Sicilian style;)

I got a call from them everytime. They always hang up when I pick up and everytime I call back they reject my call. Who is this?

me llaman y no contestan le gusta joder nada mas y gastar mis minutos

Same thing as above thanks for the info.

the number constanstly calls me and hangs up. I too have sprint, are any of you in puerto rico, I am...

dis # calls  and listen and hangs up!!!!!!like wtf!!!!!!!!

Same as all the others; and they call every single day. Is there any way to stop this? I read that it may be from Sprint. Are all of you Sprint customers?

Ok, it looks like we are all Sprint Customers, I suggest (and I already did) to call customer care (*2) and tell them about the number and ask for the "block promotional calls" feature. Maybe if they start receiving A LOT of complaints about this number they will stop.

A waste of personal and cell phone time.  How rude!

Same thing occureed to me today, I googled the number and I found this site. The courious thing is that I am also sprint costumer and is the second time I receive a call from this number.  Nobody spoke to me also and then no one answer me!

Sprint just e-mailed me back and they say that the phone number 1-888-680-1668 is not from them. I don't know if that is true or not, but the coincidences are too big. They did say however that they blocked those calls to my phone. Good luck, everyone!

exactly the same has been happening to me.  Is annoying.

This number has been calling me for the past three weeks.  It's getting annoying.  If it is sprint, i wish they would stop.  If its someone else I hope they get their a$$ beat down.

I got a call.  The guy said he was from Sprint and asked for me by name.  He had a very think accent (Spanish maybe) so it was hard to understand.  I told him to not call me anymore. That it was my cell and I did not want solicitation.  We have Sprint/Nextel for cell service.

I am also a Sprint customer. About 5 months ago I got a call from (allegedly) Sprint - also a guy with a thick accent, not Spanish though, maybe from India - about some promotion. I told him "if you have my information you should have my email address, so just send it to my e-mail"... then he "pretended" to go and get my information and then hung up. Afterwards the calls started - calling and hanging up or not leaving a message. I called Customer Service at Sprint and they "swear" that the phone is not from them. I asked for it to be blocked, and I have only got one call in the last month. Sprint knows about this website, and I believe that the first person calling is some kind of fraud, either to get yout information by pretending to be Sprint or something else. Please everybody, call Sprint, complain, and let them know about this website. This has to stop and I feel for all of you.

LOL.... now they are calling from 1-800-580-1668 instead of 680.