For successful businesses, differentiation from competitors is usually achieved by adding extra value to what they offer rather than simply competing on price. The more relevant services that can be bundled in with the core offering the more likely a potential or existing customer is to buy.
The step from discussing the benefits of a new system or upgrade to including the benefits of the leading onhold service is a small, logical and painless one. From the customers perspective it adds to the value of the proposition being offered and takes away another element they need to sort out, and from the resellers viewpoint this increases the potential value of every sale.