The cause of this problem is that UCx Server does not receive responses to SIP messages that it sends to the phone(s). After the phone initially registers with UCx, the SIP stack in UCx periodically sends the SIP OPTIONS message to each registered phone to determine if the phone is still alive. If no response is received several times, UCx assumes the phone is dead/gone/disconnected. At that point, calls to the associated extension are routed to voicemail.
To determine if the phone is considered "ok" or "unreachable", you can either
a) look at the Operator Panel page (orange = ok, yellowish = unreachable), or
b) go to PBX - Tools - Asterisk CLI and issue the command "sip show peers" - look at the Status column
Now, there can be several reasons why SIP phones would not successfully reply to OPTIONS messages. The most common one is a router between UCx and the SIP phone(s) that has the SIP ALG feature enabled (SIP ALG does not usually forward responses to OPTIONS messages to UCx for some reason). The solution is to turn off SIP ALG. If you don't have a router between UCx and SIP phones, the problem is usually a misconfigured LAN.