Default [Q] Calls cut out after a few minutes???

I've been using my Nexus 5 happily since release but over the last few weeks I've noticed that most of the calls I make are cutting off at between the 4 and 5 minute mark for no apparent reason!?!?!

I've not given it much thought as in the past I've just assumed I'd lost signal or something as generally it's been while I've been driving but I've specifically tried it twice today and it's cut me off both times at around the same time!?!?!

Has anyone else had this issue with the N5 or could it be a problem with the SIM card??

Thanks for any help...