Tips for before you even pick up the phone!
Make sure you have a list of employers to call – there is no point in calling employers that are going to be unsuitable for your candidate, challenging for your candidate to get to or with a company your candidate has no intention of working for. The easiest way to get around this is to get your candidate to do the leg work for you (you can even make it part of their jobsearch plan under SO4). Get them to bring in a list of employers THEY want to work for – that way they are fulfilling their obligations, saving you a lot of time and there will be no excuses as to why they can't attend an interview.
Do your research
It generally pays to do a little research on the company you are calling, if your candidate has suggested it for you – you can ask a bit about the role they want, if not a lot of businesses will have a website that can give you an overview of what they do (often the website will be listed in the white pages next to their number)