Hardware - VOIP Phones - Grandstream - GXP2020

FreePBX Community Forum - Grandstream phones go offline from FreePBX after registration timeout expires

The Grandstream GXP2020 IP phone does not support TLS.
Using SIPS is therefore not possible.
Even the latest firmware from 2010 does not offer TLS support.

Sources:

1. Personal experience.
2. Grandstream Forums - SRTP/TLS