You may find a shop that can carry out a repair, it could be a loose mic. See
I have an FP3 with a 3.5mm jack for headphones, my daughter has an FP4 with a bluetooth and neither of use have problems.
The network/carrier bands are important a lot of places are closing down 3G, so if you were using that take care.
You can see the bands that the phones can use via the specs so ensure you have an equivalent where you are. Expand the Network section
There is also the method of purchase. There are companies that sell abroad, but unless they pick up the warranty on your behalf you don’t get any, let alone the 5 years for the FP4