Call strange problem
 

Hi.
This morning i had three lost calls from the same person.
When i've tried to call him, on my phone has appeared the message "network busy".
He can call me and i can receive, but i've ever the busy issue when i try to call him. There aren't problem with other people, also in the same area where is the first person.
Can sameone help me?
Thanks

P.S. If can be useful, i've the CSSU Stable and the KP50.