Why won't my Galaxy S4 GT-19505 call or accept a particular mobile number?

A

AC Question

This is a problem with only one mobile number. I am unable to contact this number. It tells me that this number is unavailable and says to leave a message. It does'nt pass the message on.
When the other number tries to call me the other party is told that my number is unavailable. We have been able to contact this number with other mobiles and landlines.