Mute when I call numbers beginning 03

[swif]

When I try to call certain numbers beginning with 03..  I do get through but the party at the other end can't hear me i.e. I am on mute. I asked the party at the other end if they knew what the problem is & they suggested phone OS upgrade - which I did - or contacting my network provider. Any thoughts?

When I upgraded the phone OS, I was able to get through for 24 hours but this ability then disappeared.

Thanks for your help!

[MI5]

@swif 

At least two other have reported the same issue in iPhones.

You need to report it to O2.

Guide: How to find help & contact O2 

https://www.o2.co.uk/apps/my-network