Dispatcher described unexpected phone outage when IT initiated an upgrade prior to all personnel without being notified as appropriate.

Narrative

IT upgraded the Operations phones without notifying Operations Personnel. Phones were off-line for approximately 15 minutes. I may not have even noticed that the phones were off line; except I was communicating to an aircraft and expecting a Pilot initiated med-link patch. After the phone did not ring for several minutes; I attempted to call med-link myself; but there was no dial tone. It was then that I noticed that the phone's screen just had an 'Upgrading...' message displayed. I notified the Manager who then recalled seeing an email the day before about a possible outage. No one from IT contacted Operations to see if it was possible to bring the phones off line. IT failed to followup and make sure it was possible to take the Operations phones off line. I continued to use ACARS as my primary communication with the aircraft. It needs to be re-enforced with IT what it means to be a 24/7; mission critical department. Even small; seemingly trivial software upgrades or resets have the potential to wreak havoc when the proper notification or secondary procedures are put in place.