There’s a brief mention of the RJ11 socket in the docs for this product:
Does this phone socket allow a connected handset to use the installed SIM to make/receive calls over VoLTE (so data connectivity continues), and are there plans to develop it into working with a SIP account as a VoIP ATA?
The RJ11 port currently has no functionality and is reserved for future applications that are still under development. Its functionality will be activated through a firmware update once available.