Unlink phone numbre from imessages?

Hi,

My wife gift me a iphone 6, i used this iphone in US but when i come back to my country(Dominican Republic) i can't recieve the SMS.

My operator services showed me how i set its the SMS service but for this i need unlink the older phone numbre first.

I saw this tools https://selfsolve.apple.com/deregister-imessage for unlink the number, but the related number with imessages is at US.

Please some body help me.

Regards.