Many people keep asking me why I use Tropo instead of Twilio. It seems that Twilio does a great job of getting the word out about their service, so many people end up using them first. But there are a few reasons I'm using Tropo instead. Basically it comes down to features, scalability and support.
In addition to sending SMSs and calling phone numbers, Tropo can send messages over other text channels such as AIM, Google Chat, MSN, Yahoo IM, Jabber and Twitter, and can also call Skype and SIP numbers.
Tropo recently released Phono, a browser-based SIP client, so you can now do things like have a click-to-call button on your website which will use the computer's mic and speakers to make the call!
Phono also turns a browser into a Jabber client. Your browser gets an address you can use to send messages to via the Jabber protocol. This means you can quickly make a chat interface without having to build the server-side code dealing with sockets.
Tropo lets you set the caller ID of outgoing calls to any arbitrary phone number, which can be used to more tightly integrate with existing phone systems. For example, I'm using caller ID tricks for a client to route calls directly to extensions in their office phone system.
Tropo can do great voice recognition to let callers speak instead of enter digits. For example, you can prompt a caller to say "sales" or "support" instead of just entering 1 or 2 on the keypad. This is especially great for when you need to ask the caller to choose from a large number of options such as states, or cities within a state.
Tropo doesn't charge anything for development applications until you put them onto the production network. This is really great for experimenting with a new idea without running down your development "credits."
Twilio runs on Amazon EC2. Tropo runs in the private Voxeo network. "Co-tenancy is hard." says John from Netflix in a blog post titled 5 Lessons We?ve Learned Using AWS. Personally, voice data and SMSs are not something I want running in a shared environment where resources may be allocated away from a call while I'm on the phone.
Additionally, if you are building an application that needs to be secure, such as for a bank or a government agency, they probably won't allow the application to run on the Amazon cloud for security reasons. With Tropo, you can license Prophecy and run your own Tropo platform in a secure location.
I've had nothing but great experiences with Tropo support. They are very responsive to tickets submitted, I usually get a response back in less than an hour.
There are also several Tropo staff available on their IRC channel (#tropo on freenode.net) at pretty much any time of the day. There are also several community members that hang out on the channel and can help with questions and issues.
Here are some links about projects built using Tropo.