Best to get a bunch of sim cards and a cheap phone, bag and tag them with each account. You don't need to top them up. You can pick them up for £1. I hope this helps.
Put the sim into an unlocked phone. Dial a number. Automated voice will say you don't have enough credit and arrange a top up. Phone can now receive incomming calls.
Vodafone and ASDA sims both work like this.
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I just order free ones from Vodafone. Very easy to order them in in matches of 10 or 20 at a time, take only 5 minutes. I must have 400 of the things laying about as I just set up an automated form entry bot and leave it running for a while. Numbers are totally random as well, unlike O2 where they can be just be 1 digit apart sometimes (although this may not be enough to link you I dont risk it)