Goes straight to the answering phone

[Terry700]

I now use a GiffGaff sim card into my xperia XA1. But when someone tries to call me

it goes straight on to the answering phone with the words  " its not possible to connect your call" 

How can I fix this, because I am missing so many calls

[Strampke]

Did you contact your provider?

How is it in safe mode?

Did you test with someone else's SIM card?

[EMS_MO72]

Did you try to test your SIM card in another device and check if the same thing is happening or not ?