Why doesn't my iphone 4s let me call one person?

I have had my phone just over a month and it's been working fine till about a week ago when i found out i couldn't ring one person. It just rings once and goes to voice mail and on their phone it doesn't even register that someone has rang them. They can ring me and we can text each other though. What can i do?

Hi… Maybe some of the settings have been changed, go through the settings and reset your phone, resetting will change any ringtones you have, alarms and wallpapers, this should rectify the problem… If this doesn't then it needs a complete restore, this wil delete everything you have on the phone and change its settings as though you have a brand new phone… To save all your contacts photos n anything else use icloud available in settings to backup anything you have… Once this is done it should work, if not you should phone your service operator who will make an appointment with you in your local apple store who will either fix the problem or give you a new phone… Hope this helps