Can't say for Zoiper, but I've worked this case with Counterpath (sadly for Canada - it is no more, has been acquired by Alianza).
Anyhow, the final conclusion at the time was:
The client is correct to expect that this is the same INVITE request that took different paths to the target (as indicated by the different branch IDs) - and thus correct to reject it with a 482 because the requests are not identical in content. https://tools.ietf.org/html/rfc3261#section-8.2.2.2 states what should occur if there is no To tag
The suggested solutions were:
a) increment CSeq - impossible to do with a proxy
b) send both SAVP and AVP lines in the SDP of the original INVITE. If media encryption is set to mandatory and SIP TLS is enabled then the SAVP offer would be accepted. If media encryption is off in Bria, then the AVP offer would be accepted
Good luck.