If you have put a new SIM in your phone you will have a new telephone number. If you have given anyone your old number they will not be able to call you.
In the UK all SIM providers must allow you to keep your old number on your new SIM if you want that.
This website is a good guide to keeping your old number: https://www.uswitch.com/mobiles/guides/porting-your-mobile-phone-number/
3 step process:
1 – Get a PAC code
Keep your old SIM in your phone.
Text ‘PAC’ to 65075 (free). You will get a message back with your PAC code within two hours.
2 – Give the PAC code to your new network
Give your PAC code to your new SIM provider:
- Send my PAC to Vodafone: https://myaccount.vodafone.co.uk/settings/keep-my-number/pac
- Send my PAC to O2: https://www.o2.co.uk/help/device-and-sim-support/how-to-keep-your-mobile-number/online-form
3 – Wait and swap SIM
The network will now transfer your number over to your new SIM. Your number will be the same but you will be using your new SIM.