I'll try to keep this short...
If you are doing actual sessions from a home studio ISDN or an Internet equivalent (Source Connect, Audio TX) is an invaluable tool along with a traditional phone patch. To do Radio/TV imaging, ISDN capability is expected and necessary.
However ISDN is becoming hard to get from the Telco's in many places and it is expensive. Installation runs around a hundred and fifty dollars for the two lines necessary, plus a monthly charge of forty to well over a hundred dollars a month, depending on location. It's customary for the talent to not initiate a call, but if you do you'll also be hit with line charges per minute -- times two! A Chicago production house spends over a thousand dollars per month on ISDN line charges for audio book narrations. And there's the gear - you'll need an ISDN Codec (coder/decoder) essentially an expensive computer based modem - three to four thousand dollars new.
Enter the Internet and VOIP - Voice Over Internet Protocol. With a high speed Internet connection and a software solution you can send and receive air-quality audio via the Internet.
Source Connect is a plug in program that runs with Pro Tools or any audio program supporting VST plug ins (Cubase, Audition etc.) the cost is $395.00 and ---- that's it. No monthly fees. Audio TX in more expensive at about twelve hundred dollars - including an ISDN adapter card for your computer - but it has the advantage of being able to "Talk" directly to an ISDN user by emulating an ISDN codec and it can communicate over the Internet alone.
Now this would seem a simple decision, skip ISDN and go Internet - but like everything in life it's not quite that simple.
First off, ISDN has a big established infrastructure and is very reliable. I believe it will be remain the "mainstay" for real time professional audio production (especially among broadcaster who are LOATHE to change anything!) for at least another five years.
Internet based audio will make an impact however, as the user base grows and in time goes from being a backup to the main solution. Remember that with Source Connect BOTH parties need to have the software, high speed connection and fairly powerful computers. If the 'other end' doesn't have SC you'll need to set up (and most-likely pay for) a bridge - connecting the software to a traditional ISDN codec and on to the ISDN-equipped end user.
Secondly, because of the way data is sent via the Internet today (disparate packets of data are sent in bursts) vs. the streaming of contiguous audio, Internet audio can sometimes be choppy and have other audio artifacts. In fairness, ISDN can get a little goofy too from time to time but it's fairly rare. When (note: when) the Internet changes protocol (which it will) these minor hiccups will disappear and we'll be putting the ISDN codec up on the shelf next to the DAT machine. Meanwhile, Source Connect unveiled a new version of their software for 2008 that automatically detects and ‘dropped’ out of order packets and restores the audio seamlessly.
Right now, I'm using both remote solutions with ISDN the main player and SC a nice backup. On the road, it's Source Connect - because so far I've never found a hotel room that just happened to have a couple of ISDN phone lines installed. While speaking at the NAB in Las Vegas last year I did a problem-free session from the 50th floor of the Wynn hotel on my lap top using Source Connect ---- wirelessly! This weekend I'll be connecting with London from San Francisco during my Long Haul Workshop...it's a truly amazing thing.
So to summarize my tirade.
Everything in life and business is situational - so evaluate the amount of remote work you are doing to help you decide on ISDN or an Internet program.
If - for example - you are doing windows for five TV stations five times a day then ISDN is the only way to go at this moment. If - on the other hand - your schedule includes only occasional remote work, then go for a software solution. You may have to bite the bullet occasionally and schedule an ISDN bridge, or go to an ISDN-equipped local studio but that's still far less expensive than the cost of full-time ISDN capability. BTW many recording studios give VO's a nice price break - especially if you can be flexible on scheduling - so be sure to ask. Patches in Los Angeles, Audio Recording Unlimited in Chicago and Full Compass is New York are a few examples of 'talent-friendly' ISDN studios.
I do believe ISDN audio will be replaced by Internet audio solutions .... but not just yet!
Of the two biggies - Source Connect and AudioTX - I'm betting SC will come out the winner because of their already large client base and low cost. I've seen and heard of several other Internet-based solutions, some with monthly fees which greatly lessen their value, but - at this moment - Source Connect is well-positioned to be the de-facto standard. In my experience, their customer support is beyond amazing - I can't say the same for AudioTX.
Wow that was long -- but you asked.
Hope this helps,