experiencing very weak signal & lots of dropped calls for more than 2 weeks

have owned HTC m8 for a year & a 1/2 w/o any real complaints.
currently weak signal, dropped calls, at times no service & most of the time I try to use Cortana I receive a verbal message that
" the internet & I are not talking right now" what can I do...!!?
my phone service is t mobile service...
ty, joseph

from Windows Central Forums http://ift.tt/2gwtExi
via IFTTT