Employer's VoIP calls unable to come off of hold

My wife works from home for an agency taking customer service calls.  That agency provides the laptop (Lenovo Thinkpad L15-20u4 and headset).  We have a XB7-CM modem/router from Xfinity for our house.  She connects to it via an ethernet cable.

At some point during the past year, something "broke".  She can place someone on hold, but when she brings them back off of hold, they can no longer hear her.  This used to work with this physical setup, but now it doesn't.  She might go a few days without putting someone on hold, so she doesn't know when she lost this functionality.  It was quite a few months ago.

She's not the only one who lost this functionality, and her IT department has been helpless in fixing it.  One of her co-workers tried downgrading their modem, and that did fix the problem with work, but can't get it to work with her own personal computer ... so that's not really an option.  

If we need to buy a router and use the Xfinity router/modem in bridge mode, I can do that, but I want to know it is going to work first.  I'm not convinced that the problem would be fixed with a new router.

Because of security concerns, I can't get at the laptop to explore settings and such.  I'm betting the problem is on her agency end, not on Xfinity's, but I can't prove it.

Has anyone had this kind of problem before, and can you give me some clues as to how to fix it?