14.5.5.2. Restrictions 
The DeutschlandLAN SIP trunk Telekom supports encryption only when
both signaling and payload encryption (TLS and SRTP) are active together.
Should one of these be sent unencrypted then the connection is terminated.
On the part of the Telekom only SDES is supported for voice encryption
(SRTP). For configuration and relevant support of SDES, please see
sub-chapter “1.5. Session Description (SDES) Protocol” under “Signaling
and Payload Encryption (SPE)” from IP Solutions.
IPv6 support is still in certification phase by Telekom.