A user on Teams Phone can call coworkers fine, but calls to an outside number just fail, ring once and drop, or never connect at all. Before assuming something is broken in Teams itself, this is almost always one of a small number of configuration issues, and they are worth checking in order.
Check the calling policy assigned to the user
In the Teams admin center, under Voice then Calling policies, confirm the user is assigned a policy that actually allows PSTN calling rather than one restricted to internal calls only. Organizations sometimes apply a locked down default policy to new accounts, which blocks external calling until someone manually assigns the right one.
Confirm the user has a phone number and the right license
Under Voice then Phone numbers, verify the user actually has a number assigned, and separately confirm they hold a Teams Phone license alongside Microsoft 365. A user can look fully set up in every other way and still be blocked from external calls simply because the phone number assignment did not fully propagate, which can take a few minutes to an hour after being set.
Check for a voice routing policy or dial plan issue
If your organization uses Direct Routing through a third party carrier rather than a Calling Plan from Microsoft, external call failures are more often on the voice routing policy or the session border controller than in Teams itself. Confirm the correct voice routing policy is assigned to the user, and if you manage the carrier side, check that the trunk itself is not the one silently rejecting or dropping the calls.
Still blocked after checking all three
If policy, licensing, and routing all check out and calls are still failing, the next step is usually to pull the call quality and diagnostics data for that user's failed calls in the Teams admin center, which will show the specific failure reason rather than leaving you guessing.
Teams Phone setups get complicated fast once Direct Routing and multiple calling policies are involved. If your organization wants a hand getting external calling reliable across the board, our Teams Phone team can help. Get in touch if you want to talk it through.