Not able to call mobile numbers

What happens when you try to dial a mobile number ?

Have you stored the numbers of your contacts in international format ? I.E. +44 7xxx xxx xxx ?

As a short term solution, you can call people through WhatsApp, providing our course they also have WhatsApp. As your data is working, over which WhatsApp works, then this would be a workable short term fix. 

Also, when abroad, it’s a good idea to manually select a network, as sometimes, the network your phone automatically connects to may not have roaming agreements with Smarty. Although, as you can call landlines, this probably doesn’t apply to you right now.