CSFB Blocked Call

Hi Experts,
An interesting observation during Drive Test is that whan user A dial call to the user B both are connect to same Cell/site -> CSFB Block msg occur.
But when it call to the other position user which is not connected to the same cell/Site, CSFB happen at the time snap above for reference.

What could be the reason?
Vendor is Huawei.


Do a trace on the call.
That issues occur in sites/cell or in other sites/cell if you do the same test?

Yes, forwarded to CORE team.

Only with the same site.

Doubt this issue is related with RAN at all.
Has anyone taken CORE traces?

Yes it’s enable now.

MO means mobile originated, right ?

  1. A->B same cell: csfb nok
  2. A->B different cells: csfb ok

My idea: cells are same TAC or not, check paging issue of source cell, problem happen every calls in same cells or just some csfb calls failure.

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Yes, correct.

Problem with every calls.

How is TAC?
And how MO csfb to fixed number in faulty cell?
Did you test MO csfb to pstn fixed number?

It is fine.

Also please check TAC, and cell Paging configuration.
And number of RRC users license.

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Thank you!!
LAC TAC mapping issue observed…:+1:

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LAT TAC mapping defined at MME/DNS end.