In setting up a high available telephony system most worry about the back end and ensure it functions as they would expect and require. However one highly visible user issue I have seen is a misconfiguration of the connected SIP phones in regards to the registration timeouts. When these are very high on your SIP phone then it may not notice a service has moved (via IP/DNS/etc changes) due to a HA switchover and can potentially miss incoming calls until it does. Typically an outgoing call attempt will work or at the very least cause a registration attempt to the new server the service has moved to.
For example take a look at Aastra, their defaults in a few models I’ve seen are at a half hour for a failed registration.
If the failed registration timeout is half an hour and the phone attempts to re-register and fails your phone will show an error or unregistered for the next half hour. This can happen in the cases where the registration comes in as a box is failing or a failover happened and the configuration is being written/updated due to the switchover process. More reasonable set of values are shown in the following.
In this one I’ve lowered the registration failed retry and also the timeout retry timers. These will make the SIP phone resolve the registration issue quicker by retrying more often than the defaults. They could be lower depending on the situation.
One precaution before everyone sets these very low. These settings should be set appropriately when the SIP phone is off-site and there are protections, for example Fail2Ban, in place to block brute force attacks. In these cases where the SIP phone is on an app on a mobile device this failed registration timeout should be set high enough to not trigger a lockout of a valid device. If the devices are in-house or IPs can be whitelisted then the values can be lower without worry.