Many users of Tropo want to communicate with their app using Skype - either for testing, or in some cases for production traffic. There are a few nuances to the interaction, most notably that you will need Skype credit, as there is no longer a method to call a Tropo application as if it was a Skype to Skype call. We did have that available for some time, but Skype deprecated support for it and we were forced to let it go as well.
In the same vein, you will need a Skype Number from Skype if you want to receive a call from Tropo on Skype.
Regarding SMS, you should be able to text Tropo numbers from Skype, as long as you have credit; receiving texts FROM Tropo, however, isn't something Skype supports. The phone numbers you can acquire from Skype are essentially voice only, meaning they can't accept texts. If you sent a text to Tropo and wanted to receive a reply back, you'd have to set the outgoing "from" as a mobile number that can receive texts - per Skype documentation:
"If you want your friends to see your phone number when you send them an SMS text message, you first need to verify it. This ensures that they can reply to your SMS text message and you’ll receive their reply on your mobile phone."
As an alternative for voice calls, you a SIP client like Blink or Jabber (essentially a SIP softphone with an associated SIP Provider - Blink comes with one preset), or Google Voice. For SMS, if you're U.S. based then Google Voice should work for you - if you're outside the U.S., you'll likely need to use a paid service.