[Lexman8]

It's unclear which battery is being referred to here, not helped by the confusing names given to the batteries. Here's my understanding of them but I'm happy to be corrected.

There is the main battery in the boot which handles most of the usual functions including supplying the starter motor. Then there is the so-called 'stop/start' battery which doesn't actually feed the starter motor but its job is to ensure that there isn't a voltage drop to the various ECMs, which there would otherwise be, caused by the starter motor drawing a very large current (which would cause a voltage dip).

The OP refers to a VoC service being required so perhaps it's the VoC backup battery that's failed. This battery ensures that even if there is a complete electrical failure (following a crash, say) the VoC/SOS systems will still work. I don't know if this VoC battery is rechargeable or is an extended-storage lithium cell. If it isn't rechargeable and the OP had disconnected the car's main supply for several hours, it's possible that VoC/SOS had reverted to backup mode (as it should) but ran out of power after a few hours. It's unlikely that the VoC backup battery is rated to supply power for more than a few hours.