Well it looks like I don't have to bother as I can now use my ipDTL to dial in to ISDN...and it works just great. Tested it with five ISDN studios this week.
Regards the potential work I am missing out on, I realise that it would make sense to go for it but it's still a sizeable investment for something that must sure ...
I have been voicing full time since 2012 and have so far managed to avoid having to invest in ISDN...but it seems that many production studios still insist on dealing only with ISDN VOs.