Depending on the kit in your airplane you could always try the box 2 call i.e.
Airfield B this is G abcd box 2 working Airfield A looking to route from c -d through the overhead at 3000 ft anything to affect?.
The box 2 call tells the controller you are already receiving a service from somebody else and are just "checking in" if theres nothing to affect continue with Airfield A.
Handy if there is two of you and Airfield a's frequency is busy i.e. you dont have to call to QSY and you dont have to call back with them.
Just a thought not exactly CAP413 I know....