Steps to reproduce:
1. Open gnome-contacts
2. Make sure an addressbook is selected (it happens with both the local addressbook and online accounts)
3. Click "+" (in headerbar) to add a new contact
4. Give the new contact a name
5. Type a phone number into the given (already shown) phone number box
6. Go to "Add Detail" at bottom, and add another phone number box (can be either Mobile or Home, doesn't seem to matter)
7. Type another phone number into the newly added box -- (notice for some reason that the text of the previous number is highlighted, don't know if this is related, but bug persists even if I manually de-select the text)
8. Click "Add" (in headerbar) with both phone number boxes filled out and observe that gnome-contacts segfaults with

(gnome-contacts:...): folks-WARNING **: phone-details.vala:72: Empty phone number passed to PhoneFieldDetails.
Segmentation fault (core dumped)

9. Re-open gnome-contacts and observe that the new contact is nowhere to be found

I've noticed that the segfault only occurs when adding another phone number (no segfault when adding another email) and only when doing so on a new contact (no segfault when editing an existing one)

Some hopefully useful information:
Distribution: Arch Linux
gnome-contacts 3.14.2
folks 0.10.0
libnotify 0.7.6
dconf 0.22.0
gnome-online-accounts 3.14.2
libgee 0.16.0
telepathy-glib 0.24.0
gnome-shell 3.14.3
gtk3 3.14.6
systemd 218
linux 3.18.1