You can have one Callcentric account, which, like most SIP VoIP providers, has some way to identify you (their "account number", which starts with 1777), and that one account can have a wheelbarrow full of different inbound telephone numbers (DIDs) and/or extensions. Each such DID and/or extension needs to have a unique identifier, so it can separately register with your VoIP user agents (software phones, ATAs, PBXs and/or IP phones). That identifier is the SIP username.
In Callcentric's case, the account number is always 1777xxxxxxx, and the main/first SIP registration uses that same number as the SIP username. You assign whatever SIP password you wish, on the Callcentric portal. Each additional extension's SIP username adds a three-digit suffix to the main 1777... number, and also needs a SIP password.
The Callcentric account password can (and should be) something entirely different from the SIP password(s) used for each extension.
Example:
Joe Obidude has a Callcentric account, 17771234567, and he assigns mypassword as his account password. This credential pair is used to access Joe's Callcentric web portal, where he can manage his account settings, voicemail, billing, etc.
Joe has two inbound DID numbers. He wants one to ring on OBi SP1, and the other to ring on OBi SP2.
DID #1: SIP username = 1771234567 and SIP password = dumbpasswordone
DID #2: SIP username = 1771234567101 and SIP password = dumbpasswordtwo