How is it that my aunt was able to send me a text, but she can't receive texts from me?

Question

My aunt is a vodafone (ireland) user, while I am an Africell (Gambia) user. We sorted the issue, by having my aunt switch over to o2. We can now text away. I just want to know why this happened.
Last year, we were able to text back and forth, with the old vodafone number, but now it's no longer a two way process. Something to do with arrangements? Or other technical things?

Answer 1

They may have changed there frequence.