Introducing a new BSS to support the company rating and billing process is a complex program. That complexity needs to be broken down in smaller workstreams and multiple milestones with clear quality criteria. This will allow to detect any deviation early enough to allow for mitigation and keep the overall program on track.
Also, these programs generally span over 12 to 18 months. Over such a period, the environment changes and there will be good reasons to challenge initial assumptions and revisit the scope and content. This represents a risk of functional churn which needs to be managed to avoid a constant revamp jeopardizing the final delivery.
To avoid falling into these traps, strong program management practices must be implemented managing new requests on the way, ensuring the quality criteria are met at each step to secure the program delivery.
SpikeeLabs has a successful track record of managing complex programs of that nature, allowing for flexibility while never losing the final objective.
See also how you should manage the Change Management