An early chapter book, sold by Scholastic in the mid-1980s. A father and son meet a mother and daughter and they crash conventions together. The mom works in a call center (for a 1-900 number?). A llama may be involved.
Stump the Bookseller
New format, same book stumper solving.
Definitely Journey to an 800 Number by E. L. Konigsburg! (It was a camel!)
Journey To An 800 Number, by E.L. Konigsburg. It’s a camel, not a llama, but I think it’s the one you want.