MNP – Mobile Number Portability – is a Network Feature that allows an end user to keep their Mobile Number and use it with another CSP – Communication Service Provider. MNP is focused on Number Portability. MNP is not focused on Service Portability, e.g. Prepaid Services, SMS, RBT, IP services, etc..
For Prepaid you will need to look at the Prepaid / Intelligent Network (IN) platform and the interaction with MNP-SRF.
The Prepaid platform can use MAP V3 AnyTimeInterrogation (ATI) message to request the value of the “porting key” from MNP-SRF. The key can then be used with the Prepaid / IN platform to determine the rate for the Voice / Telephony session.