Most networks grew fast when gsm arrived with the constant addition of 'boxes' as they are called - hardware units for specific tasks. Pretty soon every box was connected to every other box and no-one understood how it worked. So then came the mediation soft layers which track everything that's happening. Any network operator can tell you by the minute how many messages are being sent and received. They have to because of the roaming deals they have with other operators, which are somewhat like the old physical post deals.

So it would be simple to implement - the government would charge the operator and the operator would pass on some, all or none of the charge to the consumer.

I can't answer about ISPs though - though I suspect that they don't track individual messages 100% - they are more interested in the amount of data packets as a flow, because the deals across the networks are more about pipelines.

You can't be me, I'm taken