Sold iPhone 5 keeps asking for voicemail password?

[RealSkyDiver]

I sold my friend my old iPhone5 with iOS8 on it, wiped everything and deregistered it on iCloud/FindMyiPhone and everything worked fine but now it keeps asking for the voicemail password? She thinks it's asking for my password but I don't think that's possible since it's not connected to my account anymore and uses a different simcard.

 

[Intell]

The password it wants is the voicemail password for the buyer's account.

 

[Newtons Apple]

It is a 4 digit numerical code assigned when setting up you voicemail. I get this sometimes when I switch to my 6 from my 6S.

 

[Applejuiced]

The password it wants is the voicemail password for the buyer's account.

Correct.
It's asking for the voicemail password of the SIM card/phone number that is inside the phone.
She just gotta call customer service and reset the voicemail pin.