Using Smarty on 3 as well as 3 home Broadband

Hi, be interested in relevant comments!  We have two iPhone SE's running on 3's network via Smarty.  We also have Home Broadband running on the 3 mobile network.   For some time, prob a good few months, we have random problems with making and receiving calls.  We call someone and the phone says we are calling them but there is no ringing sound and the recipient is not receiving any call.  Several retries and still no outgoing connection, then randomly the call goes through.  Also, we get text messages to say we have missed calls but the phones are not receiving the calls, they are silent.   The phones are fine, it is not the phones, it must be the network.   Even having wifi calling enabled doesn't help, calls do not connect out or in.  My wfi speeds through the 3 modem are very good.   My question is, would having  both broadband and 2 phones using the 3 signal cause network signal capability issues?  The modem and the phones are in close proximity so distance isn't a problem.    I like Smarty but if we have frequent  trouble making and receiving  calls, we will have to change networks.