Well basically, larger companies normally own blocks of numbers. We've come across two numbers in the same block (3555 and 3522). Switchboards are normally (but not always) on "0", so my assumption was that their switchboard was on either 3520 (again assuming that "2x" was their voice block) or 3500 (which is at the base of the 35xx block we came across earlier).
Not that I know a great deal about how businesses allocate number blocks, but you might have noticed that I haven't suggested 3550 - my reasoning behind this being that it could be their fax block. However, if 3520 and 3500 don't bring anything up, you could try that (029 2085 3550).
Hope that makes sense to you

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