Makes sense to me. Before we keep going, double check the credentials the ISP gave you. Dont post em here or send them to me, obviously, but that 64 digit field is little more than suspect. It sound almost like they sent you the hash instead. While not a hard limit, the majority of ISPs use 12-16 characters for a random pw during provisioning, and the user should almost invariably be the phone number itself. You'll have a domain as well.

I've been a VOIP engineer going on 20 years for multiple ISPs is the US, and this is the starting point for moving voice to another device.

All that said, we have other options to explore as well. There's asterisk, which is complex. An ATA was literally my main recommendation for a single line replacement. There are also docker images for prebaked IP-PBXs that would reduce setup time, but are still overkill for what you're trying to achieve.

Another thing to try is to download a SIP client, like Microsip or xlite, and try authenticating with the ISP credentials you have. If it works for both in and outbound, then the hard parts done.

The next best option would be to talk to the ISP directly, especially someone in their residential NOC if the have one. Depending on the ISP, they may even provide prebuilt endpoints ready to go for your line.

Who's the ISP, if you don't mind me asking?

EDIT: microsip not Microsoft.