What does "unknown number" mean?

A phone number that is unfamiliar to the recipient.

Unknown number

Common synonyms include: missing value, not stated, placeholder, undesignated, undetermined, undetermined number.

noun

noun ·Rare ·Advanced level

Noun

1
A phone number that is unfamiliar to the recipient.
"Kelly looked at her phone. This unknown number keep coming up. Answer it."
2
A phone number that is hidden from the recipient.
"The voice on the phone sounds familiar, but the incoming call on my phone reads “ unknown.” […] And why are you calling me from an unknown number?”"
3
Used other than figuratively or idiomatically: see unknown, number.

"Kelly looked at her phone. This unknown number keep coming up. Answer it."