Hello,
While not the cheapest, I'm with Vodafone, as they offer a booster service for calls, over WiFi (WiFi calling).
I know that means having to have broadband, but I will tell you that it works, very well. The calls are very near prefect.
I'm in an area with very poor mobile signal, across all networks, so I would be lost without it.
I think Eir are the only other company to provide WiFi calling, but be aware of their terrible customer service, before even considering them.