Strange service messages on my Doro phone

[seahermit]

I just wondered if anyone had any bright ideas about this, Richard seems to be one of the wise-guys on technology ..

Besides my smartphone, I use a small Doro "feature phone" i.e. one with a keypad. I am getting text messages which I have never seen on any phone before. Not responding seems to make no difference - the phone works well and reliably.

"Invalid Certificate Error:
A certificate has expired or has an invalid validity date
Certificate details:
Serial number

[mass of numbers, presumably a code]

Version 2
Issuer: TeliaSonera
Server CAv2
TeliaSonera
F1

Subject www.doroexperience.com
Doro AB
IT
Malmo
SF
Valid from 04/11/19"

When I click on Options, the word Install comes up - clicking again does nothing, merely produces a blank page, the message then disappears and is repeated in a day or so. Weird! Never seen anything like this on a simple little phone and it makes no sense to me whatever.