Just a thought, but mine are trained to a whistle (as well as voice and hand signals), and a whistle does get a dogs attention better than voice, particularly when the voice has been ignored. It will need training - it doesn't just happen, but a whistle command is much more consistent.

I use three pips for recall and a pup will be introduced to it on it's first day. Initially I will use it at dinner time to call a dog over for dinner. It will start to associate it with good things. Then you need to train it in all the different situations... starting with an easy situation and then working your way up to distractions.