Q: unable to text message to particular number

i am unable to send text message to some of my contacts on my 6s. 16gb

when i tried texting them.. its says "not delivered"

this is problem with particularly some numbers in my contact list.

when checked with these contacts they all using different carriers and different o.s hand sets.

surprisingly i am able to text when i tried the same sim card from a samsung and lumia hand set.

Pl help me out.