Many thanks for the response. Yes, I have inserted the SIM correctly, however there is no stage during set-up where I'm asked to enter the PIN, nor does there seem to be an option to do so subsequently (bear in mind this is not an Android phone...).
Might I snap the SIM down to Nano size, insert it in my own phone, and thereby activate? I'm guessing the 3-size SIM template will allow me to reassemble to Micro size and retry in the Doro...?!
Thanks for the prompt response. If it is possible to try the SIM card in a different handset then that would be helpful - it can help us to narrow down if the SIM card is faulty or if the problem lies with the handset.
Make sure that you use the additional size extensions that came with the SIM card. Do not adjust the size of the SIM card with scissors or other tools as this can potentially damage and destroy it.
Thank you for the advice, I have tried the sim in a Pixel 5 which I know is unlocked, it will not connect to any network (gave it 24 hours just to be sure, apologies for the delay in getting back to you).
On a few occasions I got an odd message, something like 'sim not configured for voice calls'. Also, the phone with sim installed refuses to connect to my home wifi (although I'm not sure this isn't a red herring...).
Essentially, I think the Sim must be deficient in some way, and that's before I cut it up with scissors to fit (I am joking...!).
PS Further, just tried sim in an (unlocked) P20 and 'emergency calls only'. At no point have I been asked for a Pin or activation key of any kind.