I've had a few incidents of this across several systems. There was another forum posting about this a few months ago: https://www.3cx.com/community/threads/cannot-make-internal-incoming-outgoing-calls.50391/ The systems in question are Linux installs (mixture of 15.5 SP1 and SP2 releases). All are hosted with the same provider. All phones are STUN. I start receiving alerts of calls to the system failing due to Channel Unavailable. When calling the number, it rings as a normal "busy" or with a message stating "Your call could not be completed as dialed." Reboots of the server sometimes resolve it. I've attempted re-registration of the SIP trunk (Vitelity) but still the same issue. I finally was able to resolve it by restarting the SIP services from the services page. I'm posting this here for the following: 1) Informational -- Hoping others will find this post helpful in the event of a similar issue. 2) Documentational -- I could not post to the thread linked above as it was closed for additional comments. 3) Inquiry -- It would be good to know if others have experienced similar to this and, if so, it would be a helpful datapoint for 3cx to research the cause of this issue and rectify it in a future update. Thanks.