|You almost certainly do get a different IP address whenever you connect a mobile phone. You may occasionally get the same IP address back, it depends on how big the ISPs DHCP pool is. |
Getting a different IP address every time you connect has absolutely nothing at all to do with security. It has a lot to do with sharing a limited number if IP addresses to the maximum number of users. If every registered user to an ISP tried to connect simultaneously the chances are they would run out if IP addresses and somebody would have to wait till one became available.
Slightly different with an always on connection like broadband but you are still likely to get a different IP address unless you have been allocated a static IP address which usually costs extra.
By the time you get round to entering the user name and password the PPP link is already established. The PPP link is established automatically as soon as you press connect. The user name and password is to allow you access to your ISPs network.