I don't think even Carol Ann Lee knows the answer to that riddle. &%+((£
Better write and ask him, because if he didn't then he's wasted 30 years on hold and is off-the-hook. 
He had a phone with an automatic call back function so it was funky to at least that extent. The thing is, he said in interview that the police should be able to verify from telephone records that the call was made. That means it must have been picked up at Bourtree Cottage. If there was no answerphone then there was no way of pulling off the crime so as to leave a trace of this call. He believed a trace had been left. Therefore:
(I) no answerphone = not guilty
(II) answerphone = guilty or not guilty, and
(III) if guilty, he had an answerphone
Now there is a second problem. If Bamber called from WHF at 3.00 a.m. how did he place a call from
Bourtree to Julie a minute or two later? That seems impossible but, if he believed, right or not, the call times could be retrieved then he had to solve it.