On Friday, 4 June 2021 at 12:57:21 UTC+1, Davidm wrote:

It seems that it does rely on you having mobile data on your phone
turned on.
As I'm just on a payg arrangement (not even a contract) I don't
normally have it switched on, ever since on holiday one year and
loosing £40 of credit overnight when the bloody phone decided to do a
software update over a slow link (cruise ship).

It uses the data transmission functionality of SMS i.e. it is more akin to a 'text message' (but one that won't appear in the user-facing message store) and hence requires only the signalling path of the base GSM service rather than the packet switched data extensions which, as you say, might well be turned off or otherwise not available.