Hello

I need an app for messages, my problem is one. I travel alot and i have two cards. One is when i stay in my country, i can send and call anyone and i don't pay nothing more. This is the card i use to talk with my girlfriend. But when i go to work i use a second card, in that i have a specific plan so a do not pay roaming. I send message to a second card of my girlfriend and she reply from is first card to my first card.
Basically when i not in my country i need to chang the conversation between this to numbers in sms.
I want to know if are any sms application, that helps me. But that application is not supposed to work on data.

Thankyou