I have a problem with TLS negotiation in two Zoiper versions (Zoiper 3.2.21357 64bit for Linux and Zoiper 3.6.1. for iPhone). In both versions I got "Certificate Validation Failure" for domain sip.viptel.sk.
Their certificate is signed by RapisSSL and apparently they have a wildcard certificate *.viptel.sk.
Do you have problems with their Certifice Authority or it is a problem with a wildcard certificate? Or anything else :)