Your phones don’t appear to be able to reach the DPMA service running on the PBX.
But you have a more serious issue to deal with. Your DPMA version is out of date and the built in SSL cert will expire in a few weeks. You need to get DPMA upgraded asap:
I did a yum update and it seems DPMA was upgraded to 3.4.13-1.shmz65.1.51
I rebooted the server completely.
I can connect the phones again!
However I still get the following message in the dashboard:
An outdated version of the Asterisk DPMA module was detected (v3.4.1) . Please upgrade to a supported version (v3.6.2+) to ensure uninterrupted usage of DPMA supported phones.