Text missed callers open times and book them by reply

from google.oauth2.credentials import Credentials
from googleapiclient.discovery import build
from twilio.rest import Client
from datetime import datetime, timedelta, timezone
from clickruntime import on_event, credentials, http, state

FROM = "+15005550006"

def sms(to, body):
    return Client(credentials.secret("TWILIO_ACCOUNT_SID"), credentials.secret("TWILIO_AUTH_TOKEN")).messages.create(to=to, from_=FROM, body=body)
cal = lambda: build("calendar", "v3", credentials=Credentials(credentials.connection("google", "work")), cache_discovery=False).events()

def free_slots():
    now = datetime.now(timezone.utc).replace(minute=0, second=0, microsecond=0) + timedelta(hours=2)
    busy = cal().list(calendarId="primary", timeMin=now.isoformat(), timeMax=(now + timedelta(days=5)).isoformat(), singleEvents=True).execute()["items"]
    taken = {e["start"].get("dateTime", "")[:13] for e in busy}
    hours = [now + timedelta(hours=h) for h in range(120)]
    return [t.isoformat() for t in hours if 9 <= t.hour < 17 and t.weekday() < 5 and t.isoformat()[:13] not in taken][:3]

@on_event
def handle(event):
    f = event.json or {}
    caller, choice, offer = f.get("From"), (f.get("Body") or "").strip(), state.get(f"offer:{f.get('From')}")
    if f.get("CallStatus") in ("no-answer", "busy"):
        slots = free_slots()
        state.set(f"offer:{caller}", slots)
        sms(caller, "Sorry we missed you! Reply with a number to book: " + "; ".join(f"{i + 1}) {s[:16].replace('T', ' ')}" for i, s in enumerate(slots)))
    elif offer and choice in ("1", "2", "3")[:len(offer)]:
        start = datetime.fromisoformat(offer[int(choice) - 1])
        cal().insert(calendarId="primary", body={"summary": f"Call-back booking {caller}", "start": {"dateTime": start.isoformat()},
                     "end": {"dateTime": (start + timedelta(hours=1)).isoformat()}}).execute()
        sms(caller, f"Booked for {start:%a %d %b %H:%M}. See you then!")
        state.delete(f"offer:{caller}")
    return {"caller": caller}