Why is Siri and dictation not using wi-fi when available unless "forced to" (even when wi-fi assist is turned off)?

I have iphone 6s on 9.3.1 and have an issue with Siri and dictation in particular which is driving me bonkers as I can't figure it out.

I have both wi-fi and mobile turned on (and wi-fi assist turned off). When at home I have a good wi-fi signal and all apps and internet, e-mail etc work absolutely fine over the wi-fi. I am on PAYG mobile and do not have any credit so therefore although turned on, I am not getting any data through the mobile. This is causing me no issues generally as everything is using the wi-fi ..... this is except for Siri and dictation which seem to be preferentially using (or at least trying to use) the mobile network rather than wifi. Due to the fact that I have no credit, I just get a message to say that there is no connection when trying to use either of these features (they are not even attempting to use wi-fi).

The only way round this is to physically force the wi-fi by completely disabling the mobile. When this is done, both siri and dictation work fine over the wi-fi network, however this isn't really a long term fix as I don't want to have to have my mobile network permanently turned off so that things use wi-fi. I had always thought given the choice, wi-fi would be used (unless of course wi-fi assist is switched on which it isn't).

To be honest the only reason I've noticed this is due to the fact that I have run out of credit on my PAYG mobile account. if I hadn't I wouldn't be any the wiser - however would be using as Siri would be working fine - I would just be using a **** of a lot of unecessary mobil data!

If anyone knows what is going on I would be really grateful