Mine too. All fine for about a year. Then just started going silent. Replaced sim and then had second phone sent out. Same issue, calling and receiving calls practically impossible. V little support from either carrier (o2) or samsung - who claim that it isn't a known fault. A lot of these posts seem several months old - did any of those earlier posters get a resolution.
On O2 in the UK, pay monthly business account
Same problem...next phone is an LG. Speakerphone won't stay on during calls . I've disabled almost everything in the enabled apps. Must be a hidden samsung app which causes it.