Consider your specifics! The choice of customer communication tool should be based on your product. It is better to sell complex products through call-centers and simple ones via e-mail or SMS.
The golden mean is bulk voice calls. They are not such expensive as a call-center, while they are much more likely to deliver information to a client due to higher delivery and open rates. Imitation of a real human voice involves customers to conversation and the IVR-scripts provide them more information or immediately place an order and transfer it to a manager.
The size of the contact base matters. If the base is small, you can call it manually. Voice mailings are most effective if you have bases of 1000+ contacts. SMS and e-mail newsletters, due to lower conversion, will be useful for bases of 5000+ contacts.
Always test your messages on small groups of customers. It is better to test several offers on groups of 100-500 people before sending them to all clients.