The other day at work my son was getting texts/calls using the same carrier so this seemed to answer the question,is it the handset or the carrier.Previously he was with a different carrier so couldn't tell.He is now with the same carrier so it has to be the phone ???.
Not necessarily true.
I'm not sure which carrier you are with, but in the US, Sprint has weird issues.
Triband devices (like N5), when on Sprint, do not support two separate transmission paths for CDMA and LTE at the same time. So, when the device is in Sprint LTE coverage area, it will park in the LTE only.
What this means is that the device will not be on the CDMA network (which is used for voice). So, what Sprint does is this - when it tries to forward a call to your phone, and doesn't find your phone on the CDMA network, it will check on the LTE network. If it finds your phone on the LTE network, it tells your phone to disconnect from LTE and connect to CDMA, thus enabling the call to go thru'. This is CFSB (Circuit Switched Fallback)
In some areas, CFSB is not available, and so, the device just stays on the CDMA network (and no LTE signal). Which is again, ok for voice calls.
The problem happens when CFSB fails, and calls don't get routed.
The other problem happens when Sprint is in the middle of an upgrade, you may be in an area covered by LTE (only), with no CDMA coverage. In that area, you may end up with no voice calls, and to make it worse, this may be sporadic (as Sprint tests the two networks).
I had to deal with this issue for about a month or so, so I feel your pain.
I'm not saying that this is the same issue in your case, but just wanted to indicate that the network can present different results for different devices.
P.S. you can read more about the CFSB issue for Sprint
here