SIP Application-Level Gateway (ALG) is a setting on routers and firewalls which causes the device to become a translator between the phones and Cytracom. With this function enabled, each SIP packet is intercepted and re-written. This causes issues, and the setting should be disabled.
Note: SIP is the communication used by the phones and Cytracom to register and set up calls. RTP is a different protocol used for the actual audio, and is typically not affected by SIP issues, ALG, or otherwise. One exception is occasionally audio will be missing entirely. This might be caused by SIP issues, because the SIP communication directs the audio.
SIP ALG may be on the network, and the phones work fine for a month, a year, or more. Then, suddenly the phones will not register. In troubleshooting, ALG is noticed on the network, it is disabled, and then the phones work fine. Usually, this is noticed in the initial setup of the phones and taken care of then.
The majority of phone/device issues occur when SIP packets are re-written. Turn SIP ALG off. If the device does not have the option to turn it off, then it is recommended that the device be replaced.