Man from the phone company.

S4E8 - Dinah attempts to get Dave to install a second phone line, but he is hung up on the existing phone, which does not appear in the usual records and is not a recognizable model (being a phone from a mafia safehouse). One explanation that he becomes obsessed with is that the entire building was moved at some point, which Dinah asks about on his behalf, but this did not happen. Eventually, his obstinacy leads Dinah to send Nicky and Frank in for a less pleasant form of persuasion.

DAVE: This is the darndest thing. I have absolutely no record of this phone number. I have no record of this phone. I don't even recognize this model. Where did you get it again?
DINAH: It was here when we got here.
DAVE: It's the darndest thing.
– S4E8

8. How Do You Get to Carnegie Hall?

• List of Characters