Outside of the US, it's common for landlines to not "hangup" when you put the receiver on-hook. This is to allow you to answer the phone in the kitchen, say hold on, put it back on-hook. walk to the bedroom and pick the call up on a different phone
For human to human interaction, that is great because there is a "bye" message happening out of band (voice) that would cause both parties to hangup. For computer to human, however, the computer has no concept that the person "hung up" on them. The timeout interval for that is usually 2 minutes, and it's called "Call Sup Timer."
For a Tropo app, this would effectively mean a call to a landline that supports that functionality would not return an immediate BYE - we wouldn't know it hungup right away. If the app timeout is shorter than the timeout set by the carrier, then we'd get a timeout and not a hangup.