Can you post your variables declaration?

You might have a truncation problem. Check the following code that looks the same but has an important difference.

DECLARE @ExtType varchar(10) = 'X',

@PhoneNo varchar(20) = '+1 (202) 123-9876 X012',

@intPos int

select @intPos = charindex(@ExtType,Upper(@PhoneNo))

SELECT @intPos

GO

DECLARE @ExtType varchar(10) = 'X',

--Note the change on the variable length

@PhoneNo varchar(10) = '+1 (202) 123-9876 X012',

@intPos int

select @intPos = charindex(@ExtType,Upper(@PhoneNo))

SELECT @intPos

GO