How can you talk on your fone while still being able to bbm?

kk soo i have a blackberry curve 8750 and i use to be able to talk on the fone while still being able to bbm. as of recently it has stopped working n once i hang up the fone all my msgs get sent out and all floods in. i have researched it and it says i need wi-fi which i do have. is there any reasons why this would happened and what is the Solution? how can i get it back so i can bbm while chatting on the fone?