There are (at least) three ways to configure the phone:
1. The solution provider enters the MAC address of the phone into their system which then communicates with a Cisco zero touch provisioning service that MPP firmware phones check at startup. If a provider has claimed that MAC, the phone is redirected to that provider’s infrastructure for further provisioning instructions.
2. The user enters an activation code which is again passed to a Cisco global device activation service and, if valid, the phone is redirected to the solution that generated that code. (The code is actually split into two parts: the first half identifies the platform and the second serves as a one time password.)
3. The user/admin manually configures the SIP registration and other relevant details, such as a dial plan, in the local admin web interface of the phone. There are hundreds of nerd nobs there, so this is only advisable if the solution provider has clear instructions of what to set.
In the end this is ultimately between you and your solution provider. If they even support customer-provided phones, they either need to leverage the backend services Cisco offers or provide you detailed instructions of what to set locally on the phone. I would hope it’s the former.
