When I ring certain numbers from my mobile phone, example 0845 057 9000 (that's Halfords customer service number) I get a female automated voice telling me I don't have enough credit to make the call. Then giving me options to top up my phone.
I am on a pay monthly with you, not pay as you go!
When I searched for this query on Friday, I noticed there were quite a few posts complaining about this but they were all on iphones. They were given suggestions to fix this which are not relative to me as I have a huawei phone running Andriod.
Obviously there is something wrong with your network which you have not fixed despite it being complained about numerous times.