Aastra 53i Release Note page 31

On the IP phones, an Administrator can configure TLS and Persistent TLS on a
global-basis only, using the configuration files or the Aastra Web UI.
RN-001029-00, Rev 03, Release 2.1
Notes:
1. Persistent TLS requires the outbound proxy server and outbound
proxy port parameters be configured in either the configuration files or
the Aastra Web UI (Advanced Settings->Global SIP->Basic SIP Network
Settings). There can be only one persistent TLS connection created per
phone. The phone establishes the TLS connection to the configured
outbound proxy.
2. If you configure the phone to use Persistent TLS, you must also
specify the Trusted Certificate file to use. The Root and Intermediate
Certificates, Local Certificate, and Private Key files are optional.
5i Series IP Phone Release Notes
Security Features
25