“Server Unreachable” Phone Call Issue when calling 0800 number

Hi. 

I'm trying to call an 0800 number and it is not connecting. The call disconnects after approx. 10 secs with the message Server Unreachable flashing up for a split second. I have a Remi 14C handset. 

I also find when calling my wife or when she is trying to call me it is not always connecting first time. She is also on the Smarty network. Her phone is a Samsung S21 Ultra. 

Has anyone else experienced this?