Hi not sure anyone here can help but i have a friend with a pretty curious issue.

galaxy s8, and seems after an OS update maybe a year ago or so its since been having really terrible call coverage.  calls only typically come through when in the cbd area .. outside of that ..say in Roseneath or kelburn or other surrounding areas to wellington central if you make a call to its number it goes straight through to voicemail.  moments later the phone will get a txt re the missed call and voicemail.

this is on a spark sim. on a s8 brought locally from Samsung. 

pop a 2 degrees sim in and the thing is fine. 

any ideas other than changing providers?