Thanks Jim, Graham, and Arthur for advice and support
The USB device worked well since last June, so for a non-expert 5 GBP replacement was acceptable.
I also have an old previously unblocked mobile phone with Smart SIM, set on "Roaming", which is as cheap for texting FROM Philippines to UK but costs about 20 pence / text TO Philippines. Clearly the USB "softphone" (on screen) only works if it is plugged in to the computer.