How to use mobile data (hotspot) and calls at the same time? I have a Huawei Honor 8X running Android 9 and it is pretty new - probably a year or so old. I am currently running it dual-SIM - one for my telephone number and one for mobile data. I am doing this as my telephone contract is pretty cheap, and the data contract is genuinely unlimited 4G. All phone operations default to one SIM, all data operations default to the other.

I have noticed that my phone wifi hotspot stops working when I am on an inbound or outbound telephone call. This is becoming a bit frustrating as sometimes I need to access something on the web while speaking to someone. I have done a bit of research on the web, and found that some phones prioritise voice data over internet data, but that situation may be changing on newer phones (presumably as network bandwidths generally improve).

My situation is a bit different in that I expect I have two modems (one per SIM card) and so even if this restriction still applies to phones with one card, I hope I might be exempt because I am running two.

Of course my ideal situation would be to cancel the voice contract and use data/voice on one contract. However I am happy to maintain both in the phone if I can get if that is the only way it will work. If this is considered hard to do these days - even with a modern phone - I can transfer the data card to a dongle and carry that separately.

I am in the UK.
Going by your problem I suspected that your device is Dual SIM Dual Standby, which was confirmed by GSM Arena.  

What this means is that you can use only one SIM at a time be it for data or voice unlike Dual SIM Dual Active phones where you can use both SIMs simultaneously for voice /data.
Also the user manual which can be downloaded from here does not mention Dual SIM Dual Active. 

So, contrary to your expectations you have only one modem and hence the behavior you experience. 

Only solution is to use one SIM that has both voice and data plans since your device supports VoLTE (see user manual). Be aware that if you use both SIMs on 4G/VoLTE, only one will work on 4G and the other on 2G