Default Galaxy S3 suddenly dropping 100% of calls

Hi everyone,

My brother has had a Telus galaxy s3 for over a year now and it's always been stock and unrooted. He did the 4.3 update a month or two ago and had no problems. All of a sudden about a week Ago, his phone started dropping all calls after about a minute or two. After rooting it and installing another rom, he still has the same issue. If you look at his phone while its just sitting there, it will lose all service for about 10 seconds and then come back.

Is this a hardware issue or can I install an older modem as a possible fix? He was on 4.3 so his boot loader is up to date but I'm not sure if I can flash an old modem.

Also, is changing the antenna a possible let fix?

Thanks in advance.

Sent from my Nexus 5 using xda app-developers app