Iphone 4 reception

I'm having reception problems with my Iphone since i originally bought it.

My provider (Cosmote Greece) at the beggining explained to me that they have some issues with the "new" sim cards and they will fix it.

The problem was never fixed. After 2 years i'm still having the same issue. My provider says they have to perform some technical tests to my sim card, which is what i've seen until now Missed calls to my mobile number.

They are trying to "help" by replacing my sim card, but i'm still having the same problem.

As my phone is out of warranty, i have to pay for my phone to be checked and/or replace any parts.

Another option they offer, is for me to find another Iphone 4 and perform a test by myself!!!!!!!!!????????

Please advise

Thank you,

Georgios