AngularFirestore Simple non-streaming/non-observable querying is hard. Is there a better pattern?

I'm trying to get a simple value from a record in the Cloud Firestore using AngularFire.

I've had a difficult time getting my head around RxJS and Observables, and I have mostly got there (sort of), but sometimes I don't want an Observable stream of data, I just need a single, static value, from a single record found using a non-id query.

I have gone through the documentation but can't seem to find any examples.

This code is my crude and inelegant attempt, but there must be a better pattern? Can anyone suggest a better pattern?

Please Note This code is to illustrate the non-pattern, it hasn't been coded into a project or executed so there may be some typos/errors!

import { AngularFirestore } from '@angular/fire/firestore';
import { Membership } from 'src/app/models/membership.model';

@Injectable({
  providedIn: 'root'
})
export class MembershipService {
    email: string;

    constructor(private afs: AngularFirestore) {}

    findEmailByPhone(phoneNumber: number) {

        this.afs.collection<Membership>('memberships', query => query.where('phonenumber', '==', phoneNumber).limit(1)).valueChanges().pipe(first()).subscribe(results => {
            if (results.length == 1) {
                return this.email = results[0].email;
            }
        });

    }
}