Sounds like your phone is not connected to the network when the call is being routed through @Mike-B - when you check your phone you are causing it to "wake up" and doing so connects to the network.
First thing to do is try a new sim card - the best way of getting one is to visit your local store with photo ID.
If that doesn't solve it, try putting your sim in a different phone to rule that out as well.
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here
