External ringers or strobes connect to the telephony platform separately, not through the phone itself, with your extension shared across both devices. There are analog options that would connect through an ATA or SIP options that would register directly. It’s difficult to suggest specific models without knowing the call control platform that your phone is registered to though - and your provider needs to support it. I would have expected them to give you options here.
As for the EHS adapter: if the retailer put it in writing that it will work then I’d just confirm your ability to return if it doesn’t work. The EHS capability of a phone can usually be enabled/disabled by the platform the phone is registered to; I’d also ask your provider if it’s enabled before ordering.
