When you provide services for smartphones, a mobile-first experience should be mandatory for those providers. Until recently, MVNOs and MNO in general did not care about it and the customer experience they provided was awful. If your are a MVNO or MNO and want to excel your Customer Experience you need absolutely to focus on mobile-first experience.
APN Override is a technology that many MNOs are using to provide service to MVNOs. No matter what configuration the user has her device, the MNO overrides the APN with appropriate one for the MVNO. But what happens when roaming?