I have many FreePBX systems with the Yealink T48G and I’ve never been able to figure why it seem like the phone is re registering every 3 minutes, If I’m on a call it never drops, I can be on for hours. but with no calls and the phone just sitting there I’ve timed it, the line will go grey and all light blink and it registers and back to life it comes. Its all the phones.
You either have the phone configured wrong, or your firewall is killing traffic.
Default a phone, and manually put in the extension information, without letting it autoprovision.
Does it exhibit the same behavior? Then it is your router/firewall, because no default setting in a Yealink will do what you are describing, at least in my experience.