The question arrives roughly the moment the plane's wheels touch down: will people still be able to reach me? It's a fair thing to wonder when you've just added a second SIM to your phone, and the answer is reassuringly simple once you separate two things that get lumped together: your phone number, and your messaging apps.
Your number doesn't move
A travel eSIM is a data line. It doesn't take over your phone number, doesn't reroute your texts, and doesn't ask your contacts to do anything differently. Your home SIM stays exactly where it was, in the slot or profile it's always occupied, still registered to your number, still able to receive a traditional call or SMS the whole time you're away - including the one-time codes your bank likes to send at inconvenient moments.
WhatsApp keeps working because it never needed your carrier
WhatsApp is tied to your phone number for identity, but it sends and receives every message over the internet, not over a phone network - which is precisely what your travel eSIM provides. Land in Mexico City, connect to your travel data, and WhatsApp carries on exactly as it did at home: same chats, same groups, same voice notes, now routed over a different data line than the one you'd use if you were still at your desk.
iMessage, similarly, doesn't care which SIM sent it
The blue bubbles work the same way. iMessage sends over data to other Apple devices regardless of which line is active, so conversations with other iPhone users continue uninterrupted on your travel eSIM. The one thing worth knowing: if you're messaging someone without an iPhone, that message falls back to standard SMS, which goes out over your home SIM, not your travel data - a rare moment where the two lines actually do different jobs.