How do I find people interested in talking over the phone?

Look into finding an Adult Day Care for your mom. It will help her connect with others. It will give you a break.
You might want to check with your local Senior Center. The one near me has a "Buddy System" they have seniors call other seniors just to chat and see how they are doing. This might be good for you.
Look for a Support Group in your area. Again check with the Senior Center and see if there are any in your area. Many Memory Care facilities have Support Groups and your loved one does not need to be a resident for you to attend. Alzheimer's Association has lists of Support Groups. Many Churches also have support groups or they can refer you to one.

You might like a Zoom support group for the appropriate issue (Alzheimers, cancer, etc). Contact the relevant organizations and ask about support groups.

See if there's a local church or senior center that has a phone-a-friend program. You need to make time for yourself if you're caring for your mother. It's important for you and for your mom for you to have some respite time. Consider hiring a caregiver for a few hours a week and find a volunteer opportunity where you can interact with other people.