Many new voiceover artists complain that they are not able to install ISDN lines. We are aware that BT has been trying to get rid of ISDN for a long time (other providers are available) but frankly it is very old and expensive technology.
Thankfully, as the internet is so fast these days, there are now several IP solutions.
We now mostly use ipDTL. It costs us £200 per year but we can send links to our remote voiceover talent and they can use it for free. All parties must have Google Chrome and ethernet (not wifi)
Source Connect works in a similar way but was much more expensive. They are now testing Source Connect Now (SCN) which is much cheaper. We had issues connecting initially so we gave up - but recently we had a much better time of it with one of our voice talents. It seemed to work fine - there is also a natty recording device within the software. And then a quick download button. The restriction of this is that we cannot check that recording levels are correct.
We have not checked out SoundStreak. This claims it will work on wifi (although ethernet is better) and you don't need Google Chrome. Their business model is interesting though: instead of paying a fixed fee, you only pay when the producer has a session. (For voiceover talent it is free.) That may encourage take up but may cost more in the long run if it is your full-time business.
And don't forget you could always ask your client to your voiceover session via the phone, Skype or GoToMeeting (or other conference call software.)