I know it's a brillient idea! If you hit any company in the pocket and flood them with complaints then the issue of the cabinets being too-easy-to-open will be addressed.
- It'll cost NTL to write off chargable phone calls.
- It'll cost NTL to always go and fix the cabinets.
- It'll always cost NTL the time having to handle a major upsurge in complaints.
Finally should enough evidence be gathered, see if it's possible to get the media involved and also OFCOM. Think of it. Someone might need the emergency services but can't call them in time because of a vandalised green cabinet.
Problem is that the problems are managable and within NTL's budget so NTL does not assign a priority to these issues.
Simple business logic really.