A referral is best when it comes from soneone with whom you have a relationship or something in common that lets them refer you easily.. If you don't have this, you need to invest time and energy building it. What this means is as follows:
1. During the call aside from other topics, express your interest in working at the firm and ask him/her what are things you can do to be a good candidate. Write these things down somewhere
2. Email them every 3 weeks to update on progress on those areas they pointed out, and also set up a follow up call in 2 months or so.
3. After 2-3 months of updating them and asking for their advice - assuming you have made actual progress you should ask him if he is comfortable referring you given the progress you have made or if there is something else he would like you to work on
Continue working on yourself and building this relationship until you feel you have earned their trust for a referral. It is long but it works well as people want to feel invested in your success
There are exceptions to this rule where people are willing to refer you sooner. You will have to pay attention to the conversation and how it is going to pick up on whether they would do that for you. Sometimes they suggest it themselves which makes it easier.