A Web-based hosted system through which business entities create and manage communications campaigns. The system carries out a communications campaign on behalf of a client. A campaign is a series of calls to a contact list using one or more sub-campaigns. A sub-campaign associates a list of contacts, a script, and a timeframe. An agent typically is a contact center operator. A skill group is a set of agents that are trained to handle a given script. Call pacing enhancements are implemented to allow simultaneous progress on multiple sub-campaigns under a skill group. The techniques enable control over the pacing of individual sub-campaigns when multiple sub-campaigns are running under a single skill group. The service also provides a framework by which a set of “pluggable” pacing models are provisioned and managed, preferably on a per-model basis.