What is classed as inactivity on a pay as you go (UK)?

I have a question for you the readers!  Does anyone know what each of the networks class as valid activity to keep a sim active? My Student Radio Station, recently had there Vodafone number cut off due to not topping up, and I want to ensure this does not happen again.

Do any of the UK  networks (including MVNO’s)  class reciveing a call or incoming sms as activity, and therefore will not cut you off?