Cellular signal appears good but incoming audio and mobile data poor.

My iPhone 5 has a problem with incoming cellular audio and data. Compared to another iPhone 5 with the same sim card the audio on incoming calls often breaks up into static. Also incoming data is much slower than the other iPhone 5. I've had a repair guy look at this a couple of times, the first time he sent it to a specialist who replaced a chip that deals with the signal. This had no effect. The second time he replaced the earpiece speaker, again no effect. The phone was gifted to me mid 2015. It had fallen into an oil sump on a canal boat - of all things - back in 2014 but worked fine apart from most of the screen was black. I was able to repair it by replacing the screen but also replaced the dock connector, earpiece speaker and headphone socket to be on the safe side as the parts were cheap and i didn't want to be opening it up more times than i needed too. The phone worked fine after this for a few months but then the cellular problems started to creep in. Now it's unusable for cellular voice calls. FaceTime audio calls work fine with crystal clear audio, so it doesn't appear to be a problem with the speaker. Also putting the sim in other phones solves the issue. The indicated cellular signal is good when i test the phone so at some point between the signal being received and measured and the signal being processed it seems that there's a problem. I've also done the obvious things like phone resets and software restores. Does anyone have any advice or pointers?