Here's some suggestion:
1, Make cold calls at lunch time or coffee breaks, in a public telephone;
2, Have an answer machine working with your home phone, or apply a voice mail service from Bell ($7 monthly).
3, When you have chance to arrange interview, let the potential employer know you're currently working and try to set a time good for both of you.
The human resource officers understand such situation and they'll make good arrangement. Don't worry.
Finally wish you good luck!