problem with iPhone 7+

[LauraB83]

I have the latest OS, I have reset my network settings, switched my iPhone off and on again, switched it to airplane mode and back again trying lots of different troubleshooting but my iPhone will allow some calls to come through to me, others go straight to voicemail and I don't ever have a missed call notification so have no idea someone has called, other times people call me and it has a busy tone when I am not on the phone. I have checked the area around my home and it is fine no issues with the network. I am wondering if there is something wrong with the SIM as it is rather old now having been the same SIM for quite some years. 

Any ideas what I should do? I am worried even if I upgrade the phone the same problems will occur. 

[Cleoriff]

@LauraB83 

It could be the sim at fault. You can get a new one in store where they will swap everything over for you. You will need ID. Or contact o2 via these numbers Guide: How to find help & contact O2 

Or via social media on Facebook ( https://o2uk.co/O2CFB) , Twitter ( https://o2uk.co/O2CTW) or Instagram ( https://o2uk.co/O2CIG) , they should be able to help you with this

[MI5]

@LauraB83 

Have you checked to make sure "do not disturb" is off?