phone not working "network is down"

we have 4 extensions and one extension goes out intermittently and resets itself - until now.  the phone says network is down however the date and time are current and all other phones are working.  I spent three hours with tech support and to no avail.  A ticket was put in as they believed it was a bad phone that needed replaced.  took a couple days for them to reach back out but when they called back the phone was working again.  2 days later the same thing I rebooted the phone it sat for a couple days and suddenly was working.  worked again for a day and now its down again.  cant keep running to the conference room to answer the phones when i need to be at the front door and desk