The reason is simply that phone calls across Europe were and are usually not free/included in flat rate plans. Telcos just never found a way to agree on something that makes sense: It's quite common to get unlimited domestic calls, but pay 10-30 cents per minute to a neighboring country.

Nobody (other than businesses) actually pays that; people just ended up using WhatsApp for both calls and texts instead.

So in a way it's the absence of regulation: Data roaming is free within the EU, thanks to a corresponding EU regulation; before that, it wasn't unusual to pay more than EUR 10 per Megabyte (yes, Megabyte, not Gigabyte).