iphone to android more problems with texts

I too switched from Iphone to a Samsung Galaxy S5 and ported my old o2 iphone number to 3 on the Samsung.
I then added a new sim card new number from 3 to my old unlocked iphone.

My new S5 withold number happily sends and receives texts from some of my contacts (some but I dont know if all of whom, are iphone users.)

BUT it will not send to at least some of my contacts whomayormaynot be iphone users -I get a messaage on teh S5 screen when I attempt to write a message - 'INVALID RECIPIENT - Unable to add this recipient. The recipieits number is not valid.'
But the number is valid and I am ambe to send them a text from my old iphone even though it has a new sim card and new number on it.

How can I fix or who can I talk to to get it fixed please?
Thanks
cxt