Dialling in roaming

Hello, I've noticed during some tests made with a SIM in roaming that when I called myself I see my local number showed instead of the E.164 format number. In Wireshark I see that the numbering plan is set to unknown in this case (but this happens only when I select a specific operator). I started then to dial local numbers (while in roaming) and I could call any local number, from any network, without having to use the country code. Is this something that operators do? Cheers, Bogdan