How far you want to go depends on your threat model. The higher the threat model, the more you will have to sacrifice convenience. It may be a situation where radio communication with frewuency hopping and encryption is better than using a cell at all.
Keep your phone's location services off. Either way, your SIM can track you, your phone is constantly pinging cell towers. The only way to use a cellphone anonymously is to have airplane mode activated 24/7 with no SIM card, plugged directly into a modem, no WiFi. Then use Tor, or at the very least an always on VPN with killswitch engaged. GrapheneOS will spoof your phone's MAC address.
The travel phone should be kept in a faraday bag when it's in your car and at frequented locations that could doxx you. This is not always possible for many people, but the more convenience, the less privacy.
The idea is to understand the more often you use it outside and where you use it, the easier it will be to fingerprint and track you and act accordingly.
New scanning tech allows your device to be "fingerprinted" to your tag number and registration, and all the other signals coming from your car (car cell modem, animal chip, mobile, even library books) with a scanner WHILE YOUR CAR IS MOVING.
Definitely put the phone in airplane mode or turn it off and keep it in the faraday bag whenever possible before turning into a parking lot. Scanners are often used in parking lots.
If you have to travel, silent.link has some pretty awesome pay as you go, super fast data only esims. Because they are anonymous international esims, the cell providers can see that they are on the network, but because they are roaming on the network, the network provider cannot attach that sim to your identity.
This does not however prevent device scanning or triangulation, hence the faraday bag.
You have to know your threat model. Some people with more extreme threat models may choose to use no phone or use multiple throwaway phones and multiple esims.
When travelling, carry an open source travel WiFi router like the Beryl AX with OpenWrt, OpenVPN, and Wireguard pre-installed and connect to that LINE-IN instead of the hotel WiFi.
