Media placement is just choosing what media to place your advertising in. OK, would really recommend getting some professional advice from a marketing company (not exclusively PR as that’s just one element in the marketing mix), but:
There are worse things that just asking in this sort of situation. Put together a quick proposal summing up the sort of questions you would see people asking and a background to your experience. Make sure its clear what’s in it for the station, otherwise they will probably not be interested.
Then target radio stations that your target customers would listen to - you may not know this, and this is one area the PR company would have an advantage over you. But this bit is important - advertise on the wrong station and you are wasting your time. The benefit of using the PR company is that they also have loads of useful contacts in the industry who they can tap to help you here.
If you have no clue, try asking some of your customers what they listen to. **If they don’t listen to the radio very much, you may be wasting your time. **
Contact each station and work out the right person to send this proposal to, likely to be the programming manager, but someone who works for a radio station may have another suggestion. Make sure you get a correct name and title, and submit your proposal. Follow up with a phone call. Be patient, it may take some time for them to find an appropriate slot.
However, re my point above, you have just assumed that radio advertising is the right medium for your advertising. I think you have jumped to the solution without properly identifing the problem and considering a range of solutions for cost effectiveness and response.
I think this may be a bit wasted without doing a little research as to what triggers your customers to contact you. Find out more about them - where they live, what media are they exposed to, what made them call you in the first place. If you are providing a service that people only turn to when there’s a problem, make sure your advertising is in the yellow pages etc. If you are a regular service and you need to remind people to call you, other businesses successfully use things like magnets, desk planners etc. Ask yourself what you want to achieve, and then make sure the thing you do is designed to achieve this. You may want to target both existing and new customers - if so, they may need to be targeted with separate executions.