Cisco 7961 Headsets on UC520 System Not Working, Speakerphone Works Fine

See whether the issue is related to the wire connecting the headset to the phone.  Experienced similar issues with different cisco phones, possible cause the variance in the impedance of the wire.
i.e. an of the shelf longer phone wire was used to connect the handle to the phone and they had issues, they could hear what was being said, but the other side could not hear them.

Working/notworking only one side can hear, or neither can hear.

If not mistaken, there is a special function button on these phones to activate the headset unless it is connected through the handle.

I'm working on this system remotely, so I can't see what's truly going on. Fortunately, the office manager asked for the phone number of a phone to be changed, so I can see if things work out for that line.

Both ways in the convo are fine, it's just that the headset doesn't work.

But I remembered hearing about a way to programmatically turn the headset function on and off. Was wondering if anyone knew what the might be.

Are you able to connect to the phone via its ip? 10.1.10.x for the ip of the phone, ...
Otherwise, it has to be adjusted directly on the phone.

I couldn't see anything that I could configure. But I did change the extension associated with a phone. Will give more info tomorrow.

Changing the extension is a configuration push from the pbx, the auto-on for headset is, I believe, a local phone config that can only be done by directly accessing the phone config via the IP the phone has.

When I went to the IP of the phone itself, there was nothing for me to configure. Just a display of several settings. In terms of the PBX itself, I've been configuring using Cisco Configuration Assistant.

I do not believe the CCA prvides for the controls you are looking for. You could download the phone BIN/config or using CLI though still do not think from the PBX you can force the headset function on. if not mistaken, that would be eqivalent to forcing phone off hook.
The other option, is to tie the headset as an alternative to the handle with the device that would trigger the off hook event (lift handle)......

Well, I did reconfigure the one phone last night, so I should know tomorrow whether it just may be a matter of the 2 phones themselves. Just odd for things to act so weird.

I just read this whole thing after reading your last comment and noticed I should've been saying handset the entire time, not headset.

Correct, they pick up the handset and hear nothing. However, the speakerphone is perfectly fine. Supposedly, these phones were working fine before changing the extensions that they are associated with. Calls can be made and picked up without incident as long as the speakerphone is used. Ringing, dialing, and so on were never issues.

3 phones were done the exact same way on the same day, and all initially had the same problem. However, after changing the extensions again, then back to the correct ones, one phone started working.

My changes basically compromised of 3 things, all done from CCA:

1. Changing the extension
2. Changing the name associated with extension
3. Creating a voicemail box

Office manager insists that all 3 phones worked normally before changes were made. Since I'm 3 hours out of town, I have no way to verify. However, one of the phones that still has an issue is used by an employee who has been active the entire time.

The idea of the cords did come to mind, but I had her check the button on the phones first to make sure that it's not an issue of it coming off hook improperly.

No news from the office manager, who has had some things going on that kept her out of the office. Will update ASAP.

You're going to laugh, but the office manager had reported that she got them working by hitting the handset button.

Things started working without any reasonable explanation. However, the advice provided should be helpful in some similar scenarios in the future.