The Forwarding feature in Voxalot is part of the Premium services.
Depending on how many "Registered" providers you plan to have, you have to get VoxLite or VoxPro (1 or 5 registered providers respectively).
Alternatively, you can create a SipBroker alias which you can point to your Callcentric account (check out
SIPBroker - Login).
To use Callcentric for outgoing calls, you do not need to register it.
To receive Callcentric calls to your Voxalot account, you can set up a Forward under the "Call Treatments" tab when you log in to your Callcentric account, and forward all calls to sip:xxxxxx@voxalot.com
Note that you can only register Callcentric in one SIP user agent (soft-phone, IP device, or service like Voxalot).
You have a lot of choices to achieve what you want, so you need to understand how the different services work together to achieve the specific features and configuration that will work for you.