Same issues here; just returned from Europe (used two SIM cards, one for 3 IRL and the other for France SFR).
Data, no problem, but cannot receive nor place phone calls.
Sprint's pretty terrible for international roaming, hence the local SIM cards. But to return to the 'States and have a handset useless for business use? That's.. really bad.
If this can't be hammered out without a factory reset before Saturday evening, I'll have to jump ship for T-Mobile (wife's carrier, amazing international service, by the bye).
Set Preferred Network Type (In KitKat: Settings > More > Mobile Networks - Preferred Network Type) to 1X. Perform a profile then PRL update (under Carrier Settings in the same menu). When experiencing this issue, despite informing of a successful update, it looks like the profile and PRL updates fail in LTE or Global mode.
Once the update is complete, switch back to LTE or Global. Test your calling ability.
I had a very similar problem to Maxxrox: traveled abroad, used a local GSM SIM in my Nexus 5 without issue, returned to the USA, replaced the Sprint SIM, and had data and SMS working but voice calling not working (including incoming voice in my case).
Maxxrox's solution makes a lot of sense, but unfortunately it did not work for me.
It seems that some setting in the EFS partition related to voice calling must be changed when switching to a GSM network... and was not restored properly even when updating profile + PRL.
Fortunately, I had previously made a backup of my phone's EFS partition by following the instructions at Nexus 5 EFS backup on XDA-Forums. Restoring my previous EFS backup did the trick and restored inbound/outbound calling for me. If you have the technical understanding needed to make an EFS Backup and Restore, then I highly recommend doing so before traveling abroad :-).