Sorry for the delayed answer. Just a follow up to Rawand’s response.
For Roaming to work there needs to be a Roaming Agreement between the HPLMN and the VPLMN.
This will help to ensure that the Service Level Agreement can be fulfilled and will ensure that
Billing information can be extracted / processed and sent to the relevant Mobile / Wireline operator (CSP). Refer to TAP3 / RAP processes.
With GSM (and 3G, 4G, etc..) the HLR / HSS / UDC / etc.. is updated when there is a change of Location.
This is required so that the HPLMN can signal and route calls. At the time of writing (2019) most CSPs are using CAMEL, e.g. Phase 2, as a Feature to help with Telephony PrePaid services. The VPLMN and the HPLMN will
as a minimum be required to support CAP V2 and MAP V3. The MSC/VLR/gsmSSF and the STP layer will need to be
able to process IMSI and MSISDN.
IMSI analysis and MSISDN digit decoding are Features that MSC/VLR/gsmSSF supports.
VBR/ Wallis Dudhnath
CAP – CAMEL application part
CSP – Communication Service Provider
MAP – Mobile Application Part