Assume you want a second SIM and contract. I'm not sure this can be done on your Virgin Media Oomph account.
You may have to take out a second account with Virgin Mobile. The two operate different and 789 know nothing about Oomph accounts which are handled by a different department (Usually 150 from a Virgin landline )
Someone with Oomph may correct me on this.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.
When I try to register as a new customer it tells me that my details already exist. Correct
Use my existing VM accountand get to the point it asks for the existing mobile nuber, I take this straight from the latest bill and paste into the application form which comes back "Your Virgin Mobile number is 11 digits long and starts with 07". I can confirm the number is 11 digits and starts 07.
The only thing I can think of is that the number was ported from the previous provider but it should be recognised because it is showing on the bill.