almost never.
BUT he has the boys 2 x nights a week (one weeknight, one weekend) and doesn't contact them in between.
If he doesn't have them for any reason (very rare - when either he or we are away) he will call them midweek.
Oh, and on their birthday's & Christmas if they are with me and not him.
In the 3.5 years we have been separated I can probably count on one hand the number of times that they have asked to call him - it just doesn't happen, probably because they have other stuff to do and are confident in the routine that they have as they see him regularly.
In your situation I would pick a time each day when the children can be available. To say you can't manage it every day isn't really on - there must be a time each day when you can manage it, even if you have to limit the length. The 2 & 3 yo are too small to chat for a long time on the phone anyway, but the 9yo can.
Pick a set time for skype, if there are days when you won't be able to switch it on at that time - email or text him and let him know in advance, but try and make those days the exception not the rule.
Your 2 & 3yo can have a quick chat then your 9yo can talk to him while you are putting the 2 & 3yo to bed. Make it part of the bedtime routine "come and say goodnight to daddy".
Try and impress on him that you aren't trying to stop his contact with the children but you also have to manage life yourself, and it isn't easy.
I have to say good on your ex for upping the visits to fortnightly, although I fully understand how having him in your house is probably crap for you - I am not sure I could do that myself. Are there other options to this?
Could you show him a copy of this thread (without your username)?