In reply to a post by ehteam:

Has you wife tried forcing her mobile to 2G for calls? I think I read somewhere that 4G quality can be poor.

Its usually the other way round... 2G calls sound like an old miles long landline from 1980s, and 3G or 4G calls sound crystal clear. EE has had some issues recently, and O2's 4G calling has been temperamental. When 3G goes in 2024 (this year for Vodafone) all calls on mobile will be over 4G.

If my brain remembers correctly 2g is completely unencrypted - not that 3g or 4g much better (weak encryption).

2G has something but it’s long broken.
4G uses AES I believe. Not sure if key is broken.

3G don’t know. It’s going.

Paul's results using Three are very impressive. The speeds are excellent, although the latency is higher than on a fixed line (ours is about 14mS).

Thanks for the information. When you think of the costs of the fixed line providers now, and then compare with the availability of SIMs for £16 per month or less including unlimited data and calls, it seems to be that more people will move to mobile broadband and the fixed line providers will lose business.
It seems to me there are many other advantages of a mobile router. You can take it with you when travelling, you don't have to have a long contract, so can swap to a better deal quickly. I was also interested to note there were a couple of occasions when you ran slowly or were down. But fixed lines go down as well and BT Openreach can be slow in fixing. It took 4 days to fix our last outage.