This excerpt taken from the TWER 8-K filed Mar 19, 2007.
Revenue Recognition. Revenues are recognized at the time access to our services is made available to customers. Contractual arrangements range from one to three years. Deferred revenues are recognized as a liability when billings are received in advance of the date when revenues are earned. Our revenue arrangements with multiple deliverables under Emerging Issues Task Force, EITF, 00-21 are deemed to be immaterial.