Full disclosure - This is related to a homework assignment question. I am not asking you to do my work for me, I am merely looking for some pointers and considerations to direct me in my further research.

I have an assignment I'm working on where I've been given a scenario where a business wants to look into transitioning to using "Internet Telephone" as opposed to a traditional PSTN/PBX system and I need to write a report on it. I'm after some high level pointers from people, especially anyone that has been involved in a real life transition of this nature, on what some of the most important considerations are.

These can be financial considerations, initial setup considerations, ongoing administrative considerations, quality of service considerations or anything else that is pertinent to performing such a transition.