Yep, that's the only reason I can think of for these calls, if genuine - someone wanting to work out by when she might be missed. If not genuine, they support a narrative wherein she never went to the PoW - but mysterious people knew she intended to and must have intercepted her.
Strictly speaking it's less valid. While a business issue with this couple looks plausible given that SJL had apparently welshed on a project with them and he then went bankrupt, we still need to show that either of them was near SJL that day. The problem with that is that we don't know where she actually went. There is evidence she intended to go or went to 37SR or to the pub, but none that indicates she had to go to Stevenage Road, or was meeting anyone there, or indeed meeting anyone at all.
Her car being found there isn't evidence of this, given that it seems likely someone else drove it there. The seat position suggests this, and so does her possessions being left behind. The driver might have delayed the finding of her car by taking these out of it, but a male would attract attention if he walked off carrying a woman's hat and purse.
She apparently had a cancelled appointment with PSS that day, but there's no evidence that it was in fact kept, nor is there any that places the builder husband anywhere near her that day either. If SJL could have been meeting the couple, she could have been meeting anyone at all, on the same evidence, i.e. none, unfortunately.
The trouble with this case is that the absence of officially-endorsed evidence for anything except a visit to 37SR means that any solution you like can be projected onto it. As a result, people have speculated that "Mr Kipper" was Jewish (because a kippah is a Jewish prayer hat), was Fred West because John West because kippers, was the Suffolk Strangler because he'd possibly met SJL, was JC because he might have met SJL, and even that he was Jimmy Savile, because Jimmy Savile was evil.
Part of the reason I favour DV's theory is that SJL was an outstandingly good-looking woman beset by admirers. I have trouble with her simply vanishing unnoticed by anyone; people noticed her everywhere she went. Unless she went from her car into the cellar of a pub, I'd have expected someone to notice her out and about in the streets.