You can't run either the Test-CsPhoneBootstrap or Test-CsClientAuth cmdlet against a particular user's SIP address in Microsoft Skype for Business Server 2015 that's installed on Windows Server 2016. For example:
Test-CsClientAuthentication -TargetFqdn <FeFqdn> -UserSipAddress <UserSIP> -UserCredential <UserCred> -Verbose
The cmdlet fails and you may receive an error message that resembles the following:
"Unable to perform authentication of credentials. Inner Exception:ComputeSignature failed."
To fix this issue, install the January 2019 cumulative update 6.0.9319.537 for Skype for Business Server 2015 and Unified Communications Managed API 5.0 Runtime.