ipDTL is subscription based at $25 a month (or $160 annually) for the voiceover package. It works in the same way as Source-Connect in the sense that it connects audio over the internet. This is not dissimilar to Skype in principle except the quality is far superior (320kbit/s Stereo). This is what justifies the price as opposed to being free. The main benefit of ipDTL is that it runs through your internet browser (they recommend Chrome). You simply log into your user area and connect to the network of ipDTL subscribers.
If you opt in for Link+ ($50/month) you can use ipDTL with people that don't have a subscription by simply sending them a web link. This can be helpful if clients want to listen in but not as common for VO talent. They offer a range of packages including ones that allow you to connect to ISDN see which can be helpful when dealing with studios that don't currently have an ipDTL subscription.
ipDTL has not been around as long as Source-Connect but currently more studios and VO talent seem to prefer it since it costs less (initially) and has the same quality and capabilities.
At IVG we recommend this software or Source-Connect.
More Information: https://ipdtl.com/