Cant get the whole picture of VoIP

Hi fellows, I am currently on my CCNP track (and a bit bored with it by the way). Anyway, I decided to jump into VoIP for a while as there are more VoIP jobs than routing/switching ones around here, and seriusly Im sick of reading the wireless subject matter.

I have been reading some of the CCVP books trying to get the whole VoIP picture, but they seem too stretched out and the subjects seems scattered so I cant seem to grasp how to set up a basic VoIP network; probably because Im too stupid for VoIP

Here are my questions.

VoIP phones look for a DHCP server upon startup, is this DHCP server included in the CallManager, or should I configure a DHCP server on my own (like using a router or a MS Windows Server)?

Also, VoIP phones require a TFTP server to download the configuration upon boot, is this tftp server included in the CallManager too? Or should I install the tftp separately?

If a Gateway is connected to a PSTN, how are the off-site calls routed through that Gateway? using a ".T" route in CallManager?

One thing thats bugs me the most, why do Cisco VoIP phones use SIP?. Shouldnt they use MGCP?

sorry if the questions are lame, feel free to flame.