In regards to USA cell phone  companies .In the event of cell companies(Tmobile) not issuing signals(sunsetting) to old cell phones  will the call forwarding still 

be active, even though the phone is obsolete!

 

I have my  US number forwarded to a google voice number  while in Thailand!

 

 

I think its more about the cases when say a 2G phones (frequencies themselves) are removed for reallocation to other uses... but when that happens the mobile can only work if it also carries 3G etc...   hereinafeter swap the above from 2g - 3G... 3g/4G as appropriate

 - but in any case if the phone doesn't have the appropriate carrier (freq) available, then nothing will happen... no indications etc...