There is a true zero potential. You can detect this because two charged objects with zero delta between them will still repel each other.

I think a circuit should mostly care about the deltas, but when you’re talking about things like phone lines, the earth becomes part of your circuit. You can’t influence its potential (it’s almost exactly neutral because any charge imbalance gets removed by interaction with the interplanetary medium) so everything else is going to end up being determined by what you need for their relative potential to that.