Upper Heyford RASZ?
 

Was it mandatory to call them, or was it optional?

I'm fairly sure it was mandatory, and for good reason - their F-111s carried nukes!

I had to contact when passing, in the early 80's.

It was actually named the UHMRA or Upper Heyford MANDATORY Radio Area; does that give you a clue?.
The original RASZ only had the status of a MATZ but after a visit by a high ranking person, pressure was brought on the government to make it compulsory to request transit clearance.

It could be difficult to get a clearance. I once had to fly there in a Puma HC1 (which was always short of fuel). Having called up for a VFR entry clearance the controller held us outside for 45 minutes. Just as I was about to declare a fuel emergency he called us in and then tried to climb us and give us vectors for an ILS. I then had to break the bad news - the Puma didn't have ILS.

I well remember the controller(s);
'GXXXX Upper Heyford QNH ZZZZ saypositionheadingandaltitude'.(Yes without gaps)

A case of “Please talk slower - I can’t listen that fast!”

As a relatively new PPL in 1976, I called up Upper Heyford when transiting from Birmingham to Oxford. They gave a turn for ident and promptly forgot about me! Ten minutes later I requested to resume own navigation and received a half-hearted apology as I got the map out, trying to work out where they had vectored me.

If memory serves you didn't ask for a clearance to enter, you just had to make contact, it was really the forerunner of the RMZ and worked quite well. You could fly through without restriction but ATC had a good idea of what was in the zone.

istr that despite not requiring clearance to enter they'd nonetheless belt out a string of machine gun-fast gobbledegook without a pause between words and bossily try to vector you half way around southern England to avoid aircraft in the next county. It was simplest to just tell them where you were routng VFR and get on with it, but they clearly didn't like - and were unused to non-compliance to their rather ridiculous 'avoidance' measures.