I'm interested in your reasoning. The real strength of cloud based services for IVR is in their dynamic scaleability.
For 25 agents you can do your IVR inhouse on a spare low-mid spec machine with no licensing and only the electricity to pay. For example, I know of a 40 seat centre that uses a two year old dell laptop for their IVR, Predictive dialler, Broadcast campaigns and the Call Recording for three DDIs. It handles all of their blended campaigns and CRM too.
Call costs from 1/4p per minute, mobiles from 2.8p to 4.5p depending on time of day and network. No tie-ins, licenses, monthly recurring costs or minimum contract terms on any of the equipment or services!
Can you please walk me through your decision to use cloud computing ? Is it just for future scaleability ?
If your agents are in-house it would be a cheaper option in most cases to host an Asterisk box inside of your network perimeter. A decent machine will easily handle 100 call appearances, and far more if you enable re-invites or match the codecs to avoid a transcode operation.
If so, then how quickly are you planning to scale beyond... well, lets say 60 agents to be safe.
Alternatively, If your agents work from home then your cloud computing option is likely more viable, but still not a given. And, even in this case, I'd suggest throwing Asterisk into an Amazon EC2 instance rather than paying through the nose for hosted service. Bearing in mind that in a cloud hosted solution that's almost certainly precisely what they will do anyway, except they will charge you a monthly fee for the privilege.
-TheITGuy