When keeping up with ex work colleagues for networking purposes, when is the best time to call them?

Is it best to call people during the work day, at home time, in the early evenings, or at weekends?

Is it best to email them first, or is a phone call OK?

If they are not your friends then it would be best to do it in work time as they might not want to deal with work matters out of work time. I would say email first too however it depends what you are discussing with them. Never be pushy, but always be on your mark.