Yes - you do not need to use their router at all for SIP. You also don’t need to do any weird configuration wrt. firewalling/forwarding/etc. To use the HyperOptic-provided 020 number, you just need to get the hostname, username and password they provision in their router and you can just email support@ and they’ll tell you.
Most likely, your settings will look like this:
HYnnnnn (nnnnn being a five digit number matching some of your billing data)
<randomly generated blurb>
You can just configure it in a SIP handset like this:
If you need any help with that let me know (for advanced use, you can also put it in e.g. Asterisk and have an answering machine which emails you the recordings and sends known cold callers to Lenny - I used to do that and listening to the recordings’s always hilarious).
But I digress - price/quality wise HyperOptic is the best deal available in E20, and they’re really helpful if you email them on support@ or call them (they even answer within hours on e.g. a Sunday evening)