ipDTL: 
If you will primarily use just a web browser on your computer to make, receive and record high quality audio calls, then ipDTL is for you. You can add the ability to send a link to guests, so that they can connect with you from their web browser.  You can also make and receive SIP and phone calls.

Bronze allows 1 concurrent call
Silver allows 2 concurrent calls
Gold allows 6 concurrent calls

Full feature comparison

hybrIP: 
Like ipDTL, but specifically for managing phone calls for a talkshow or similar.

sip.audio: 
If you're planning to use dedicated SIP enabled hardware or apps to make or receive calls with radio stations, production studios etc, then sip.audio is for you. Get your sip hardware here.

If you will only ever need one SIP Username, then choose Individual on this page. If there's a chance you will add more, then the other option gives you flexibility, as well as Remote accounts at a reduced cost.

You can add phone numbers to your subscription, and assign these to your Usernames, as well as enabling the ipDTL Send a Link feature, to allow calls to your SIP device(s) from guests using a web browser.

Username Types
Studio: Full functionality, including the ability to call Remotes, and make or receive phone calls.
Remote: Cannot call other Remote accounts, make or receive phone calls, or connect with third party SIP providers.