Freeze one failed walk and the exact two-base system
Record one authorized call with timestamp and timezone, handset identity, SIP account, calling direction, two M400 base identities, firmware, PBX or service, starting location, movement path and first visible symptom. Preserve the M400 configuration, registration state and synchronized logs before changing radio or SIP controls.
Confirm that the deployment is the documented two-base M400 dual-cell topology. A repeater, an M900 system or two independently configured bases create different synchronization, registration and handover evidence. Keep a dropped established call separate from a later inbound call that fails to reach the handset.
Establish serving-base and synchronization evidence
Identify the serving base before movement, the intended target base, the selected synchronization source and the measured relationship at the failure location. Snom attributes dropped connections and failed handovers in multicell systems to weak synchronization-source RSSI and identifies -75 dBm as its troubleshooting floor. Treat that value as vendor diagnostic guidance, not a universal coverage or design guarantee.
The datasheet publishes ideal-condition range figures while the administration guide warns that metal, current lines, water pipes and environmental conditions affect placement. Repeat the path with doors, lifts, shelving, people and normal interference represented. Signal bars and base reachability do not prove synchronized handover capacity.
Separate DECT handover from SIP registration movement
Mark when the handset changes its serving base and whether the established DECT call continues, drops or loses one media direction. Then inspect the SIP account independently. Snom documents that roaming between bases can cause the new base to register the same account; successful radio handover does not prove that the PBX accepted the new registration.
Correlate base and PBX clocks, Call-ID, dialog state, REGISTER, de-REGISTER and Contact values. Do not call a registration replacement a handover failure, and do not call continuous SIP registration proof that the radio and audio path stayed intact.
Resolve multiple-registration support and contact ownership
Record the M400 multiple-registration support value and the PBX or provider policy for more than one Contact on the account. Snom documents that this control changes whether the first base sends a de-REGISTER before the next base registers. Compare that configured behavior with the observed sequence before changing contact limits or expiry values.
If the service permits one binding, determine whether an old Contact remained selected, a de-REGISTER removed the active path or the replacement REGISTER was rejected. If it permits multiple bindings, confirm which Contact received the next INVITE. Do not broaden an account policy merely to conceal an unresolved topology or firmware problem.
Trace signaling, capacity and both media directions
For an established call, verify dialog continuity, codec, RTP source and destination, sequence continuity and both listening directions across the walk. For a new inbound call after roaming, follow the selected Contact, INVITE, provisional response, alert, answer and media separately. A 200 response, ringing handset or stable registration does not establish two-way audio.
Keep handset limits, M400 codec-dependent call totals, repeaters and external trunk capacity separate. Reproduce with one controlled call before adding load. A failure that appears only near the published limit requires serving-cell, codec and active-call evidence rather than an assumption that every enrolled handset owns a call channel.
Correct the first divergence and retest restoration
Correct only the first demonstrated boundary: base placement or synchronization, unsupported topology, inconsistent firmware, registration-control mismatch, PBX contact policy, rejected REGISTER, stale Contact, codec capacity or the later media path. Preserve rollback, credential protection and the original evidence interval.
Retest stationary calls through each base, one controlled walk in both directions, one new inbound call after each transition, both media directions and return to the original base. Record actual results and timestamps. This workflow is documentation-derived and was not reproduced on an installed or laboratory M400 system.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- Snom M400 Admin and Installation Guide ↗
- Snom M400 current datasheet ↗
- Snom dropped SIP registrations while roaming ↗
- Snom dropped connections and failed handset handovers ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.