Lebara to lebara - free minutes catch?

thor

I saw a stand up in town offering Lebara sims with one of the tempters being that a £20 top up will get you 500 free minutes to other lebara sims. That is all well and good but I thought that once topped up lebara sims could call other lebara sims indefinitely or at least within fair usage limits. I have a couple of them from the quidco £10 free credit offer a few months ago and was going to top them up with the intension of making use of free lebara-lebara calls but won't if you need to top up 20 quid a limited number of free minutes.
Has anyone been using these free minutes? and if so how much do you get?

grumbler

http://www.lebara-mobile.co.uk/shop/offers#offer4

Your FREE call minutes and SMS will last for 30 days.

http://giffgaff.com/index/pricing

Calls, texts, call forwarding and video calls to giffgaff..........Free

You need to top-up at least every 3 months to receive free giffgaff to giffgaff calls, if you don't other UK rates will apply as above. The free giffgaff to giffgaff calls last for 60 minutes per call, so you need to hang up and re-dial if you want to keep talking.