SMS texts going to my old phone via home WIFI

I updated my android phone, but the reception where i am staying is real bad so i put the SIM back in the old phone, but kept the new phone as I am moving soon and it had better apps and camera etc. Now when i receive a text it is going to the phone without a SIM and going through the home WIFI, even when i send a text from the phone with a SIM the reply goes to the other, I didn't realise this was even possible, is there anyone who can tell me what to do, Thanks