Hello
@Kazzan and welcome to Talking Point.
Conversation with someone with advanced dementia can become very difficult because they lose the thread of what they are saying, cant process what someone else is saying and lose the art of back and forth exchange of information - often they rely on social tropes like "how are you?" "what are doing these days?" and "how is your your mother?" without being able to follow the replies, or else completely dominate the conversation. Later on they start to lose language itself and say less and less. Phone/video calls also become more of a challenge as they lose the understanding that they are talking to an actual person. My mum used to talk into the phone, rather than to me and obviously had no idea that I was at the other end.
All of this makes your situation very difficult.
I agree with the advice not to tell her that her husband has died. She will forget your answer, but retain the feeling that something terrible has happened, so will keep asking - only to be devastated anew each time she is told. Therapeutic untruths (aka love lies) are the way to deal with it. Tell her that her husband is at work, or gone shopping, or whatever she will accept. This will relieve her anxiety and not cause distress.
When you talk to her try encouraging her to talk about things from her past - how did she cope with young children? what was her best dress like? did she go dancing? or even - what was he own school like? Get her to tell you the old family stories, or even start them off yourself to trigger the memory. You could also try playing some of her favourite music over the phone, particularly if there is some memory associated with it. When I visited mum, one of the things I used to do was read poems from one of mums books of poetry. She had had to learn them at school and could still recite them from memory with me. Maybe this is something you could try.