The impression I got Unimobiles is that they do not have a dominican
republic number but using a uk mobile roaming to make calls. To the
original poster they can either get a local simcard than you can do
what unimobile suggested and use one of those call through services to
call them at a reduce rate. Second option is if incoming roaming is
cheaper than outgoing rate you can call their mobile, at the moment can
use
http://www.discountdial.co.uk/index2.php
to call the number for 1p a minute.
The last option is you setup a call back service, where they ring say a
london number it rings once, they than hangup and it than ring back
either their mobile, (still get charge incoming roaming charge +
callback charges) or hotel number than they get a dialtone when they
answer, which they can dial out from. I think if they are just on a
holiday probably the best option (is go on a international traveller
discount plan if they can for the rest of the trip) and you call using
discountdial to their number. They call you initially on say your
mobile or sms you to make you call them. Or goto local mobile shop
there and get a local simcard (if possible) and if phone is unlock and
than call that number direct using a call through service that
unimobile given you already