Actually, a large on-net customer base doesn’t matter all that much. The two things that matter for this discussion are network latency and operational performance.
If two mobile networks are using different IPX providers then handset-to-handset latency will be slightly higher because of peering. But considering all other elements that add to latency, peering hardly contributes at all. Naturally it is important the IPX provider uses the power of direct routing on an MPLS network, and has presence in at least all three global public peering points. Digital Interconnect certainly ticks those boxes.
Regarding operational performance: in case of technical issues there will never be more than two IPX providers involved (by GSMA recommendation, IPX transiting is not allowed). Operational interaction between peering partners is very good; most have been working together for many years and operate within strict operational SLAs.
Digital Interconnect has been working with other GRX providers for over 15 years and we manage to solve roaming issues with peers about as quickly as roaming issues between on-net networks.