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Windows 11 January 2026 Update Broke Remote TAPI? How to Fix ‘No Modem Available’ for LAN TAPI Clients
What changed in January 2026
Microsoft’s Jan 13, 2026 cumulative updates (for example, Windows 11 KB5074109 and Windows Server 2022 KB5073457) explicitly removed several legacy modem drivers (agrsm64.sys/agrsm.sys/smserl64.sys/smserial.sys), and Microsoft warns hardware depending on them will stop working.
In affected LAN-TAPI deployments, the server-side TAPI lines can remain usable on the server, but client dialer apps may fail with “no modem available,” even when the “Windows Remote Service Provider” is installed and lines are assigned in Active Directory.
Why remote TAPI may be failing
A plausible driver is hardening around Telephony service remote operation, because remote TAPI uses the Telephony service’s RPC interface (“tapsrv”) and, by default, that interface is restricted to local callers unless server mode is configured.
The same time window also includes a patched Windows Telephony Service vulnerability (CVE-2026-20931) described as an “external control of file name or path” issue that allows privilege escalation over an adjacent network, which aligns with Microsoft tightening behavior around remote-access paths.
Practical checks (fast triage)
- Confirm the issue is remote-only: if TAPI lines work in the server’s dialer but not on clients, it matches the reported pattern.
- Re-run client association using the supported tooling: Microsoft documents using tcmsetup to set the remote TAPI server list for clients, and notes it requires admin rights.
- If you must restore service urgently, some environments reported that uninstalling the January patch on the telephony server restored Microsoft Remote TAPI behavior (treat this as a temporary workaround due to security tradeoffs).
Notes to keep it accurate in your article
Avoid claiming the modem-driver removal is the root cause unless you can verify those specific removed drivers were in use by your telephony stack, because the reported failures also occur in setups using LAN TAPI providers where the server lines still function.