Cannot make a call to an internal extension

a caller A cannot call an internal extension B, but caller A can make all other internal and external calls to other extension. When another caller C calls B the call went through.

Here is the error from the full log(when caller A attempts to call B)
X-Asterisk-HangupCause: User alerting, no answer
X-Asterisk-HangupCauseCode: 19

Any help will be most welcome thank you very much in advance.

The phone was rung but no-one answered.