Where are they calling from?
JavaScript (ES6),  75 73  71 bytes
Saved 1 byte thanks to @Shaggy
Saved 2 bytes thanks to @Neil

s=&gt;/1|7|(2[^07]|3[578]|42|599?|50|6[789]|8[0578]|9[679]|.)./.exec(s)[0]

Try it online!