As part of our Sippy Softswitch v5.3 we will be introducing support for provisional responses as specified by RFC3262.
The Softswitch is capable of sending PRACK message when requested by caller UA and recognizes and confirms the PRACK messages received from vendor.
The softswitch relies on the presence of RSeq: SIP Header in the provisional response when deciding whether to negotiate it reliably. This is to help increase tolerance for external UAs that might not use this approach.
A per-environment configuration option exists which allows to send to all vendors either 'Require: 100rel' or 'Supported: 100rel' SIP header in the outgoing INVITE.
The Sippy Softswitch uses the recommended by RFC3262 behavior and waits for acknowledgement for each and every reliable response before sending a next one.
More information on PRACK can be found here in RFC3262.