Unable to call one specific number using my iPhone

For some reason, my phone will not connect to one number in my phone book. Others have no problem connecting and I can phone everyone else in my contacts, but just not this number.

A message reads back that there’s insufficient credit, but I have a contract phone and it’s up to date. I also haven’t blocked the number, and he hasn’t blocked mine…any ideas?

[Re-Titled by Moderator]