Voice Over IP Questions?

Where I work, we have been using VoIP for the last few years. The pros:
1. Works good when working properly
2. Lower cost than some of your analog phone providers

Cons:
1. Server problems...if your ISP is having troubles with their servers, it will effect your IP phone
2. Phone goes out when the power goes out.
3. Some cell phones can interfere with your IP phone and computer speakers causing a buzzing noise. AT&T cell phones cause havoc on our IP phone systems.
4. Sound quality can be bad, sometimes can hear your voice echoing

This is all I can think of right now. Hope this helps!