MO calls already start with MSISDN as Calling Party Number (A-number) and generally do not require any changes.
Only in the case of multi-MSISDN can home routing be required using CAMEL to rewrite the A-number to the desired one.
A MVNO never has MSRNs because it does not have MSCs to which MT calls should be routed.
It may instead have various other types of numbers for landing or services.
If it is MNO then it will have its own MSRNs which if it receives inbound roamers it will communicate to others under the conditions below.
If it does not have inbound roamers then the calls to MSRN will come only from their own GMSC and the rules are internal.
A MNO can tell you its own MSRN range, if those numbers are inaccessible from normal telephony and / or may require special routing.
It is common for an MNO to allocate special calling routes for:
MSRNs, possibly each MSC with its range and route (so that the MNO does not have to route between them, to get there from the first place)
MO calls that are home routed by local rules set by the MNO
Separate Billing - Many billing platforms require separate physical trunks or addresses to separate MVNO / local roaming from interconnect billing
Local / national roaming = the MVNO subscriber is in the MNO network with which he has a contract
Roaming (generic / international) = the MVNO subscriber is in a different network than the MNO with which he has a national contract
Interconnect = call between different networks (A and B are not subscriptions of the same operator) regardless of roaming