Using SCTP (aka SIGTRAN) to send SS7 between softswitches works fine assuming that you know the terminating device is an SS7 enabled softswitch, and that you are not interested in services provided by SIP. By using SIP instead, a softswitch can talk the same call control protocol to other softswitches, PC clients, gateways, IP phones, and so on. Furthermore, the softswitch does not need to know the identity of the terminating device ahead of time. In real networks, it will be unlikely that the originating softswitch knows. Calls will terminate in networks owned by other providers, in which case the type of terminating device cannot be known ahead of time. SIP-T is ideal in that the extra ISUP information carried is ignored by non-SS7 devices, so it works for all devices.
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SIP FAQ : 
Is there a SIP interoperability certification? How can I test interoperability with others?