Unable to make calls or send texts on PAYG

I have an intermittent issue with making calls, which has occurred on-and-off since I transferred my number to ID mobile some years ago. Sometimes, when I try to make a call, I am met with the “you are unable to make calls as you are out of calling credit” message. However, when I check on the app, I still have plenty of credit left (and I haven’t received any recent low balance warning text messages from ID). If I attempt to call 7777 to check the balance over-the-phone, it tells me “we’re experiencing some technical difficulties at the moment and is unable to tell me my balance”.
While this is going on, I can also still use mobile data (4G) as normal, except I am unable to use the ID Mobile app (or visit my.idmobile.co.uk) while on mobile data. They only work on WiFi - thus I wouldn’t be able to top up if I didn’t have a WiFi connection to use instead.

I contacted the live chat about this today and the rep insisted that my account showed a balance of £0.00 (while the app was still showing me approx. £3.50). However he did suggest the following:
1. Airplane mode on
2. Remove SIM
3. Restart
4. Insert SIM
5. Airplane mode off
This seemed to temporarily resolve the problem, and I could make calls, check balance via 7777 (with an amount which matched what I saw in the app), and use the ID Mobile app on data, but then this all stopped working only a few minutes later.

Obviously popping my SIM every time I want to make a phone call is not a real solution (and I haven’t actually been able to get this to work again since anyway - I think it may have just been coincidence). I have also tried the same sim in another phone (an iPhone 11) but with no change.

Is there anything else I should try or is this an account issue on ID’s end beyond my control? I do remember having some minor issues when I first brought my number across so maybe my account was never provisioned correctly in the first place?