There is another possibility.
Has the number you called been ported from another mobile provider in the recent past?
Numbers remain allocated to the original provider even though they are ported out, & it takes a while from the time of the number port for the relevant charging databases to be updated.
The number also has to be an "active" account, so if an outgoing call or text is not made from the number for some time the calls become chargeable.
VM BB TV Landline. Vonage 2nd line. Freeview/Freesat HD, ASDA/Tesco PAYG Mobile. Customer since 1993
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click Mark as Helpful Answer or use Kudos to say thanks