why can i use my phone with no sim and cant use another with the sim

[Kazaksteve]

I bought a new sim card and popped it in one of my phones, all worked good.
then I changed it to another duel sim phone, again all worked good.

I then changed it back to the original phone, but it didn't work although there was a signal, meanwhile for some reason the phone without the sim card continued to work sending and receiving calls with the number?

any ideas?

 

[B. Diddy]

Welcome to Android Central! Really basic question, but I have to ask--are you absolutely certain you swapped the correct SIM back to the first phone? Since the second phone is dual SIM (and assuming you had another SIM in there to begin with), is it possible you moved that second SIM, which perhaps the first phone isn't compatible with?