1 Monty: So to stop Hitler's brain from being resurrected we stop looking for Excalibur and start tracking the Crook of Osiris.
1 Prof. Jones: Right!
2 Monty: We need to track down Ginny and Erwin. Miss Thoroughgood might have a lead.
2 Prof. Jones: Her calling card says she lives in San Francisco.
3 Monty: That's a long way from Cornwall. She might not even be there. Can we make a telephone call?
4 Prof. Jones: Whoa there, Junior. A transatlantic and transcontinental phone call? What sort of unbelievable magic is this?

Although telephones were around in the 1930s, the technology to provide connections between phones a long distance apart was in its infancy. The first transcontinental phone calls were made in 1915, following construction of cable lines from New York to San Francisco.

Also in 1915, speech was transmitted for the first time across the Atlantic Ocean, by radio. But this was not a telephone call. That had to wait until 1926, when a two-way phone call was made between the USA and Europe, again using radio to send the voice signals. Commercial transatlantic phone service began in 1927, using this radio system.

It was not until 1956 that the first workable transatlantic phone cable was laid across the sea bed, enabling reliable phone communication between the US and Europe. The cable, named Transatlantic-1, or TAT-1, was capable of carrying a massive 35 phone calls simultaneously, and remained in service all the way up to 1978.

So yes, making a phone call from London to San Francisco in the 1930s would have been... perhaps possible, but so difficult and expensive that it could have been virtually unthinkable. Any readers old enough to remember what making an intercontinental phone call was like in the 70s or even 80s can try imagining how difficult it would have been 50 years earlier.