Loss of sound during voice calls - iphone 5

i recently decided that my 6+ was just too big and started using my old iphone 5 again having shelved it in perfectly working order in December 2014.

i now find that when i make voice calls i cannot hear the other party and they cannot hear me.

facetime calls work fine as does music and other messenger apps.

i've tried disabling bluetooth as suggested as a fix elsewhere and checking all other settings....

any suggestions?

cheers!