If you read their details you will find that Zen fix your price from day
one and guarantee never to increase it - or at least they did.
Per the calls abroad as you note WhatsApp (Android user so can't speak
for Facetime) is effective and free, but if those abroad have a computer
of any sort have you considered trying Zoom?
The final option is to set up a free VoIP account with one of the UK
organisations and use that to call their landline number. Sipgate will
charge you 1.9p/min for most of Europe to a landline, Voipcheap have
many destination countries - mainly Europe but some others - that are
free to lnadlines subject to a total of 200 minutes in any week (not
sure if that is a calendar week or 7 days rolling.) If your overseas
relatives have Interweb access you should use Sipgate as
Sipgate-to-Sipgate calls wherever they are are always free. I use it
from my mobile when caravanning in Europe and have access to free wi-fi.
Most VSP's - certainly Sipgate - give you a standard UK landline number
straight off that will accept incoming calls from anyone anywhere.
For VoIP you need either a analogue telephone adapter (ATA) into which
you plug a bog standard UK (DTMF) phone, or you get a SIPphone, either
off eBay, which you programme up. If you can find one a Cisco/Linksys
PAP2T (not the n suffix version) - note branded, not a Chinese copy -
they are very reliable and quite easy to set up. The Cisco SPA122 is
effectively the same thing. Grandstream area also effective. If by
chance you have a Gigaset DECT phone system using a G0-Box 100 base
station unit, that box can handle both landline connection and VoIP but
takes a little more setting up. There is loads of on-line help in
setting them up.
For a SIPphone look at Grandstream and Yealink as two of the most
commonly available. Panasonic also make them - I have a redundant one
here that I got for my late f-in-l. Just beware that as many of these
are 'system' phones they often expect to get their power supply up the
Ethernet cable (known as Power over Etherenet or PoE) which would
necessitate buying a small PoE power supply but there are dozens on eBay
for less than a tenner.