TLS/TCP won't be the issue here. SIP is only for 9.0+ as the 'voice' component wasn't available back on 8.1
On a couple particular builds, I found that using an IP address instead of a FQDN would stop the app working on iOS. Android devices worked fine. Also, IP address worked fine if only connecting from a local network, but as soon as I moved it to the public IP, it would fail.