Query taking non-consistent time to execute
One possible explanation:

The first time you run your query it's probably not in cache, which means it has to be read from disk and stored in memory.  If your query remains the same and execution plan doesn't change, the time it takes to execute the same query will be less. 

If the SQL Server you are using is on your own computer and/or others will not be affected, you can clear the plan cache with the DBCC FREEPROCCACHE to see if the execution times remain constant. 

You can check to see which queries are in cache by running the following command

SELECT text,objtype, refcounts, usecounts, size_in_bytes,
    disk_ios_count, context_switches_count, pages_allocated_count, original_cost,
    current_cost
FROM sys.dm_exec_cached_plans p
    CROSS APPLY sys.dm_exec_sql_text(plan_handle)
    JOIN sys.dm_os_memory_cache_entries e
    ON p.memory_object_address = e.memory_object_address
WHERE cacheobjtype = 'Compiled Plan'
ORDER BY objtype desc, usecounts DESC


If you see the query 

SELECT [customer].[Customer name],[customer].[Sl_No],[customer].[Id]
FROM [company].dbo.[customer]
WHERE ( Charindex('9000413237',CAST([company].dbo.[customer].[Phone no] AS VARCHAR(MAX)))&gt;0 ) 


Then you know its in cache.

If clearing out the cache doesn't make the execution times more consistent, please include the actual execution plan, by selecting CTRL+M before executing your query.

Another possible explanation:
Since you are testing from different computers, there could be additional network latency involved when SQL Server sends the data back to the computer you are testing from. You can verify this by using task manager and opening up the Networking Tab when running your query from another server. 

Update 15:26

If bandwith was an issue , one solution would be to find out:
1. How much bandwith exists between your clients and server
2. How much bandwith you are actually using

Your network administrator or sysadmin can tell you how much bandwith exists between you and other machines within your domaine. Using Process Explorer or perfmon, you can easily determine how much of that bandwith is being used by SQL Server and other processes. Just keep in mind that when you are executing a query on the server, the results don't have to travel across a network cable as they would with a remote client, which would change execution times. 

Your inital question though is why you are seeing inconsistent execution times for the same query. So a discussion about networks and bandwith would be off topic.

Update 09:27

Here are two screenshots of Process Explorer that will help you find the graphs that will allow you to analyze, CPU,IO,Memory and Network traffic.

Main Screen

 

Graphs