Invalid sim card showing on phone since,I've joined smarty cannot make calls

Hi Stewart_2774991 👋
When you say you can't make calls are you still able to text and use data? Are you in the Midlands? Is the make/model of your phone supported by SMARTY? Are you saying the SIM card is 'Invalid' because you can't make or receive calls or is the an 'Invalid SIM' message?
I ask this as SMARTY recently turned off their 3G network. Unless your phone has the capability/ability for WiFi calling /VoLTE and it is enabled then you will not be able to make/receive calls (but you can still text and use data). 
If this is the case then it may be down to the reason I have outlined in a post I made with regards to this problem......
https://community.smarty.co.uk/t5/chats-hacks/can-t-make-receive-calls-not-necessarily-a-network-problem/m-p/28408#M18064