SimTether — your SIM, in a cheap phone. Your life, on your real one.

Your SIM stays home.
You don't have to.

SimTether turns a cheap Android phone into a bridge for your registered SIM — SMS and GSM call control arrive on your main phone, end-to-end encrypted. No account, no ads, and your messages never touch a third-party server.

end-to-end encryptedno accountsno ads · no trackersworks without our serversopen source

How it works

Two apps, one QR scan, and your SIM lives in a phone that can sit in a drawer.

step 01

Drop your SIM in a bridge phone

Any inexpensive Android phone works. Install SimTether Bridge, put it on your home network, and leave it plugged in — it becomes a small appliance for your SIM.

step 02

Scan once from your real phone

Install SimTether on your main phone and scan the QR the bridge shows. That one scan pins the encryption keys — pairing is done, forever, until you unpair.

step 03

SMS and calls, where you are

Incoming SMS and call events reach your main phone over the local network. Answer, reject, reply, send — the bridge does the carrier work.

The path your data takes

Everything between the two phones is Noise-encrypted. The optional relay only ever sees ciphertext.

bridge phone (your SIM) your main phone ┌───────────────────────┐ ┌───────────────────────┐ │ GSM network │ │ SMS inbox + replies │ │ SimTether Bridge │ ────▶ │ incoming calls │ │ (free app) │ │ dial / answer / DTMF │ └──────────┬────────────┘ └──────────▲────────────┘ │ Noise IK · X25519 · ChaCha20-Poly1305 │ └─────────── LAN / hotspot ──────┘ └─────── optional ciphertext-only relay ─────┘

Local mode is the default — traffic never leaves your network. Remote access is opt-in on both phones, and the relay can be yours instead of ours. Our hosted relay is a best-effort service; the app works without it, forever.

Two builds, honestly labeled

Same encrypted pipeline — the rooted build adds the one thing Android withholds.

Standard

Full SMS forwarding, call notifications, and call control — answer, reject, dial, DTMF. Carrier SMS only — RCS chats can't reach the bridge, so keep chat features off in the bridge's messaging app. Works on any Android 8.0+ phone, no root needed. The Google Play listing is on the way — until then it lives on GitHub Releases.

Rooted bridge GitHub only

An experimental build that can additionally capture live GSM call audio and stream it to your main phone. Requires a rooted bridge phone and is distributed via GitHub Releases only — Google Play doesn't permit it.Currently in development.

Who it's for

Phones that can't stay registered abroad. In some countries, a phone that isn't locally registered loses cellular service after a grace period. Leave the registered SIM — and its registered phone — at home; carry whatever phone you want.
A second SIM without a second phone in your pocket.Verification codes, carrier messages, and calls on a number you don't want to carry hardware for.
Privacy-conscious users who want SMS forwarding without handing message content to a cloud service.

Your SIM stays home.
You don't have to.