Hi Experts!!
Our network is merger of 2 operators, where Operator 2 user sim can access operator1 though spid definition.
We have recently shut down all 3G network (U9 and U21) in a particular city, now operator 1 sims can do proper csfallback to geran but operator 2 sims csfallback to 2G is failing.
What can be possible causes for this?
We have done lac-tac mapping w.r.t to 2G, we have ICR being allowed also for operator 2.
Any other check points?
Vendor is Ericsson.
Did you check the neighbors configuration?
Can you please elaborate?
What all thing to check?
Check the neighbors are properly created and also the declaration of external cells.
I’d compare the neighbor relations with operator 1 and 2 and trying to find a different configuration.
Please check the neighbor list.
Get the neighbor dump from OSS and compare.
99.99 % it is missing neighbor list issue.
Check if upto tier -2 neighbors, co-located neighbors are defined properly or not.
Eutran to Geran neighbors are getting created by anr, no issues.
Geran frequencies are also defined.
I have seen few cases where neighbor get deleted by ANR sometimes.
Check the neighbor list in 2G and 4G neighbor list and see if neighbor list is matching.
You might have 4G neighbor list populated properly, but 2G neighbor list might not be properly defined.
Check the priority of neighbor.
Agree, this may be the issue. 
Sorry, I think I could not explain my issue properly, let me write this way:
I have operator 1 and operator 2 merged and gradually becoming single plmn network after consolidation. Now operator 1 sim users have no issue latching to enb and doing csfallback, but the other operator 2 sims can access the neiwork, do HOs in LTE, conduct srvcc.
Only issue with csfallback.
And this is happening after 3G shutdown when we have shutdown 3G in operator 1, and removed 3G definition in SPID list and also changed csfallbackprio, connectedmobilityprio, cellreselectionpriority, voiceprio with increased weightage to 2G.
Is this issue with all Operator 2 sites?
Can you check Logs and find out Failure Causes?