Why ringing the owner's cell from the truck is not a backup plan

The default "answering system" for most small trades shops is the owner's cell phone forwarded from the business line. It is the worst kind of backup: it feels like a backup, but it is exactly as reliable as the owner being free to talk - which is to say, not very.

While the owner is on a roof, under a sink, in a crawlspace, or driving on the highway with the radio up, that forwarded ring is silent. The customer hears six rings and then voicemail. Same outcome as no forwarding at all, with extra phone bill.