Lifting handset has no audio - Cisco 7970/7960 SIP recent issue - multiple affected out of nowhere?

Hi everyone! Hoping someone has an idea of what’s going on.

I’ve been using FreePBX for years with a few SIP flashed Cisco 7970 and 7960 phones. They have been working extremely well with the odd issue that ended up relating to the line cards and not FreePBX. However, recently (this week), in a span of two days, 3 separate phones began to develop a strange issue.

When lifting the handset, there is no audio to both parties, but switching back to speakerphone works. I initially thought it was a hardware problem with the phone, but a second phone exhibited the same symptom. I chalked it up to coincidence, and replaced both phones. A third phone has begun to exhibit the same symptom now, which tells me there is more going on.

I wasn’t able to find any information specific to this one, but does anyone have any idea, or has seen this before, why when lifting the handset all audio dies completely, and what would have caused this to happen like this?

FreePBX with Asterisk 11.7.0.

Phone firmware:
Cisco 7970 SIP: SIP70.8-5-2S
Cisco 7960 SIP: P003-8-12-00

This is an isolated internal network, so there are no firewall or connectivity issues that should cause this as calls remain within the network using physical POTS lines fed from Digium line cards and other phones remain functional (for now and more I hope!)

So this sounds like a pretty common problem with these phones I have several that have this issue. The problem that Is actually in the hardware of the phone itself I have found. When you life the reciever off of the hook the switch to tell the phone its off hook does not make good contact and the phone acts as though it is hung up. When you push the speaker button you get a dial tone because that takes the phone off hook. look at the icon next to the first line when you life the reciever without audio does the phone change to the phone off hook icon? If ya tap the hook switch multiple times hard it usually will make contact and then work. You can disassemble the phone and put a little bit more tension on the hook switch spring so that it pushes harder against the silicon contact switch, however dont put to much tension on it or it will push the receiver up when the receiver is hung up.

Good luck.