Which is more cost-effective, calling or texting?

Although SMS is cheap, sometimes things are unclear. I don't know when the other party will receive it or when the other party will check it. It is not cost-effective not to tell the delivery time of the picture.

You can form a cornet with several drivers, and you can call for free if you pay. But only if they are all moving. It is not clear whether Unicom has this service.

Or use VoIP, which is much lower. But the signal is unstable.