From the 24th of February 2021 it is no longer possible to register new phone numbers at OnlyDomains, but dont worry, your current phone number/s can be managed here.
To set up a redirection in general:
1) Log into your user account dashboard
2) Go to "Phone Numbers" -> "My Numbers"
3) Click on the number you wish to edit redirections for
4) When the page finishes loading, click "Manage Redirections" at the bottom of the page, or the "Redirections" button at the top.
5) Click "Create Redirection"
6) Fill in details as appropriate.
In most cases, the redirection type will be "telephone". When entering the destination number, please be aware of any leading 0's -- for example, to redirect to the New Zealand cellphone number 027 555 1234, you would select "New Zealand" as the country (which fills in +64 for you) and then the number without the leading 0. (275551234)
If the redirection should only be effective during certain hours / times, or if certain numbers are calling, you can enable this accordingly using the toggle switches. (Otherwise, the redirection will be used for all numbers calling, at all hours of the day.)
Multiple redirections can be created -- from the "Redirections" tab, you can adjust the priority of each one so that numbers are tried in a particular order or sequence. (e.g. call your cell first, and if you don't pick up, redirect to another number.)
If none of the destinations pick up in time, the call will go to a voicemail -- this will be sent to you by email, and you can adjust where it goes from the "Voice Messages" tab.