Need basic understanding on how this works

i just need to understand how the load calculated on * server for remote extensions, for example i have an Asterisk server in location “A” most of the users will be connected remotely , all lines using VOIP from Broadvoice . does the internet speed or the voice codec has anything to do with asterisk or it will be just for extension registration and pass through the calls to BroadVoice? also if i would like to use a low bandwidth codec like G.729 on the server and Broadvoice support it in one of their locations, is there any load on Asterisk server?