I'm currently using ID Mobile, which is a Virtural Network (MVNO) that uses 3's network for capacity and coverage. Just been with them for a month now and so far I'm happy enough with the service, especially as they offer WiFi calling/VoWiFi and 4G calling over 3's "extended" network coverage which can reach many areas its traditional 3G/4G network can't.
Saying that, all network will have black spots of coverage and within NI coverage for 2G/3G & 4G is essentially "complete" for now as the big concentration for networks is now 5G - however its coverage in the north is very limited at present.
All the main networks offer WiFi calling (VoWiFi) and 4G calling (VoLTE) but only 3 offer it for prepaid/PAYG customers, Vodafone, O2 and EE only provide it for postpaid customers with a handset or sim-only, and in all cases a compatible handset is required to make use of it. VoLTE will offer better 4G coverage than standard for EE & 3 customers as they can use lower frequency spectrum than is used by them as standard (800MHz as opposed to 1800/2100MHz). The only virtual networks I know that offer VoWiFi & VoLTE are BT (via EE) and ID Mobile (via 3).
No way has EE got 60% usage in NI - O2 probably has, Ofcom used to publish statistics of the share of users of mobile networks in NI until about three years ago and O2 were often over 60%, EE were often around 10%.