After porting my number my phone doesn't ring for incoming calls

[Riayn]

I recently had my number ported over from Vodafone and all seemed to go well, I can receive SMS messages and use FaceTime & iMessage and my number is showing up as the number that I ported over.  The only issue is when someone calls me my phone doesn't ring nor does it register a missed call. The only reason I even knew someone was trying to call me was when I got an O2 alert SMS message that someone on my contact list had tried to call me.

Does anyone know what might be causing this issue? 

[MI5 · Accepted solution]

@Riayn 

Turn off call alert and reset your voicemail timeout.

All in this guide Guide: All you need to know about O2 Voicemail 901 

[Riayn]

Thanks so much MI5.  I have implemented the changes you have suggested.  Let's hope it fixes the problem.