The reality is Collection agency relationship needs to be manage like all your customers and suppliers. There are multiple reasons why you need to take good care of your relationship.
1) If you get into a problem situation and you need there assistance. Such as a year end cash push or a big account that you need to find out quickly how it's going to turn out.
Having this relationship helps you get thinks done more efficiently.
2) Developing a good relationship let's you get inside information sometime that can be very precious. Imagine finding before anyone else that the team who has treating your portfolio has just left for the competition. Or that sales people keep coming and going. This is as much an indicator for your supplier or customers. A lot of movement in the sales force usually means a product or service that is not moving or an internal team that can not deliver on the promise of the sales rep. Either i am sure you will agree that is not good news for you .It's a classic sign of a declining performance to come.
3) Even though no one likes to get solicited for work from collection agency, the fact remains you need to keep appraise of the industry changes and method. I see this function more like a business development approach. After all you never know when the next greatest agency will come knocking at your door. Taking the time to talk to then and find out what makes them unique may make the difference between meeting or not your cash target at year end.
I usually make it a point to be direct . Don't waste anybody's time here. tell them if you have business to give to them or not . position the call or the face to face interview early on. if your not in the market for a new agency then say so but always leave a door open. I typically always tell them i have not opening right now but i am looking for who has the next great idea on how to collect. What is their unique value proposition and can they support it or is it just a nice way to sell but they have little substance. Always be correct with your sales people after all if it does workout for them at the agency your interviewing they could find work somewhere else and then call you to tell you they just found what you were looking for. If you play it straight with them they will do the same with you . At least that is what i have found to be the rule.

Also one last thing i always tell them there only as good as their last collection results and that a good relationship works both ways. they need business and i need results.

Controlling your cash is a serious business so make sure you deal with serious people.