I have tested them once also inherited from a CM site and they worked but I would not use them for Faxes, only for phones.
If you need Fax over IP you better buy a Patton M-ATA, small and powerfull SIP ATA with full T-38 fax support, works always and is a "set and forget" device.
It registers as a SIP endpoint which is also licensed as a 3th party IP Endpoint.
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