I've never had an issue in the somewhat limited time I've been using ShoreTel. The stability of the software is very sturdy, and relatively easy to use. I don't really have issues regarding the availability of the software from my place of work.
Since ShoreTel is phone switch dependent and not server dependent, the availability is great. As with any hardware, it is good to reboot the phone switches and server from time to time to prevent issues from cropping up. In 13 years, we've had two switches have fan failures that ShoreTel replaced promptly.
We have been using the ShoreTel solution for 14 years. We have gone from primarily analog phones to all IP. We have upgraded switches at least twice, and the server three times (we are now on a VMware based Windows 2012 R2 server). In that time we have had fewer than 6 outages, most of which simply required a server reboot to resolve. Our most disruptive outages have been the carrier, not ShoreTel.
ShoreTel works through a partner network. You can only buy ShoreTel products through a certified partner whose job it is to sell and support. The partner has major incentive to provide top quality customer service and aid to the customer when things go wrong, parts are needed or more assistance is required.
Call control and set up is dependent on the ShoreGear switches with fail over available if there are available resources. Redundancy is accomplished by adding extra switches (N+1), no additional licenses are needed. With 14.2 switches can now be virtualized as well which further reduces the cost of having a disaster recovery plan in place.
MiCloud Connect (formerly ShoreTel) is a business communications solution that combines a distributed communications platform, intuitive user applications, IP phones, and an easy-to-use management system that integrates with leading business information systems. Voice Switches are purpose-built appliances providing VoIP telephony with reliability and availability. The appliances help extend system capabilities and secure access for remote IP phones and SIP service providers.
ShoreTel was acquired by Mitel in 2017; according to Mitel, they will continue to offer and support ShoreTel products, now under the name MiCloud Connect.