If any MSC calls you on the phone, they need you more than you need them. I log into the job board and see what they are posting the shop for. I have had one MSC even call me and offer
LESS than the bonus on the self assign board. Then if I come to an agreement on price, I ask them to adjust the shop and assign it to me while we chat.
This assures that I get what I negotiated and it also may give me some time to shoot the breeze while the scheduler logs into the system and build up some relationship. If they are not willing to log into the "system" regardless if it's Shopmetrics, Sassie or a proprietary system and assign the shop to me while still on the phone, then they really don't want the job filled badly enough. They must do this and there is no time like the present.
If they say, I need to log in and such, say I have time to stay on the line. This is where you get to know a scheduler and can sometimes go off topic and build good will.