The padlock indicates if the audio is encrypted, so in your case SRTP is in use.
There is no visual indication what transport(UDP,TCP or TLS) is used in Zoiper Classic and in Zoiper 3, but:
- when selecting a transport type, Zoiper will try to establish a connection with it to the server. If it cannot it will not "fall back" to another transport type. This means, if you select TLS, it will either be used or it will fail and the account wont register.
- Wireshark can be used to check on the fly.