Grid computing can exploit distributed, underutilized or not, resources to provide massive parallel CPU capacity. Load balancing, applications sharing, as well as geographically dispersed databases features are other Grid's aspects which are of interest for a telecommunications operator (Telco). Building a Grid middleware in order to implement Telco's services is thus a way to assess the validity of this type of architecture for future applications. To achieve a trustworthy platform, the middleware needs to take into account accidental or malicious faults which can impact different resilience aspects. This paper describes a secure and highly available architecture which, besides traditional Grid middleware functionalities (resource broker, job mapping, system monitoring, ...), makes use of fault-tolerant mechanisms (process duplication, failure handling, ...) to guarantee QoS defined in the service level agreement. Security is carried out by analyzing each node's defense capability issue and finding a suitable solution to match this with the appropriate user's job.