Moving PAYG sim to new (to me) phone

[Stephen-g]

I have had a very simple phone for a couple of years with an O2 PAYG sim on the classic scheme. I've now acquired a "hand-me-up" old iPhone SE that has passed through a couple of generations of my partners family. I thought I could take the full size sim out of my old phone and extract the appropriate smaller size and put it in the iPhone to make it work with the existing phone number.  The phone number shows up but I can't make or receive calls. I've turned off most of the cellular data features as I don't want to use that much, although it appears that data does work, it is just the phone functionality that doesn't work.

it is now about three days since I moved the SIM from the old phone. Any hints?  Or do I have to find an O2 store - there aren't any near where I live and it will mean a train journey if it comes to that. 

[Stephen-g]

Thanks again.

I've just tried calling the phone again this morning from my land line and for the first time, it rang!  Obviously its a bit difficult to tell whether I was actually able to talk to myself (although some people think, probably correctly, that I do that quite a bit.  However, I tried again and got the - not possible to connect message.

It has just occurred to me, that perhaps there just isn't enough signal here.  My old simple phone (which I think was 2G - because I know that I couldn't use a sim from three for it) seemed to work OK in most of the house, but I'm only getting one bar on the iPhone and in a few places in the house it shows "No Service".  Although the coverage maps seem to imply everything is hunky-dory, I know (as a scientist/engineer for my working career) that radio propagation is far less predictable than the models imply.

I know my partner seems to get coverage with three here.  I know that vodafone is poor as that was the carrier used for work phones and we had lots of problems locally.  I don't know about EE.  My partner ended up with three because it had cheap PAYG rates.  

[MI5 · Accepted solution]

@Stephen-g 

Switchthe signal to 2g only in the network settings to check that.

[Stephen-g]

Well trying 2G produced no signal at all!

However, since my last post I have been out and about a bit and when out, I can phone home and leave messages and also receive texts, so I think it is working.  It is just that the signal at home is rather weak.  I did go to the top of the garden and leave a message on the landline answering machine from there as a test.

Therefore, I think it is all working - just that O2 is not too strong a signal when I'm in and around the house with this phone.

Thanks everyone for your help.