Q: I tried to switch SIM card for my iphone 4 but now original SIM wont let me use the number

Hello! So i have recently disconnected my att phone service and for a few weeks i have still been able to text and communicate using wifi which allowed me to use my old number. Today i was told about a phone service by the name of h2o which is ran through att but i was unsatisfied with not being able to use normal iphone functions so i replaced the new h2o sim card with my previous one. Upon doing so i was directed that iMessage was "waiting for activation" and no longer gave me the option to use my previous number like i had been doing before. All i want is to be able to communicate with people using my number like i was before, and i would like to know if there was a connection problem when i switched services or is this just no longer avaliable to me! Thank you!