Not receiving calls

I haven't been able to receive calls on my Smarty number for a while now. Anyone who tries to call me goes straight to voicemail. Text messages come through several hours after they've been sent. I also usually only get notified of receiving a voicemail by text at least 3 hours after the call took place, often more.

I'm using two sim cards in my android phone, and the other works completely fine, but because that one is for a foreign number there's many things I can't do with it that require a UK number exclusively. I've tried things like resetting my network settings already, to no success. Any advice would be very much appreciated!