I do understand your question but don't have the answer.
But... slightly off at a tangent but possibly relevant... I used to do the same but found that because I ended up with a network that had high call and data costs, the result was that I actually ended up paying more per year for my bursts of occasional use than I would if I got a decent ongoing tariff with lots of usage available (most of which I don't use).
I'm currently with Lebara, which is via Vodafone's network and pay £4.50 per month. But this includes 3GB of data, 1000 minutes of calls and a load of other stuff I never get close to using. But I'm still paying less than I used to pay for my occasional bursts of use with EE. Plus it's basically unlimited, and I really like having an always-on connection that I don't have to worry about.
My OH uses hers even less, and she has a lower ongoing tariff with 1p Mobile that costs £30 per year. This is more limited for data but still enough for her, and it includes unlimited calls.
Also, as a result of us both now having unlimited (or near enough) outgoing calls, we cancelled the £5/month calls package we used to pay for the landline and we don't use it for outgoing calls any more, as we both use our mobiles. So the saving on the landline calls covers a major chunk of our mobile bills, probably more now as I guess it would be more than £5/month these days.