CPA communication with a prior tax preparer

Is it ever appropriate for a CPA tax preparer to communicate with the prior CPA tax preparer in order to gain an understanding of a proposed tax engagement?

Consider this situation: Prospective tax client presents his prior tax returns with a request to prepare the current tax return and all appears to be in order except that the invoice from the prior CPA preparer is substantially less than (in the example case, 1/5 or the typical fee) the normal prevailing fee. An inquiry to the client does not reveal any useful explanation. Apparently the client thought this was the normal fee. I offered to withdraw from the engagement, saying I can’t compete even if I charge several times more what they were paying. They want me to continue but offer no explanation of why they want to switch accountants or are OK with the much higher fee.  The logical thing seems to be to get their authorization to speak with the former accountant. The client agrees to grant this authorization.  But should I make this inquiry? And what is the likelihood of this effort providing any useful outcome?

The AICPA Statement of Standards on Tax Services (SSTS) #3, Statement #2 says: “In preparing or signing a return, a member may in good faith rely, without verification, on information furnished by the taxpayer or by third parties. However, a member should not ignore the implications of information furnished and should make reasonable inquiries if the information furnished appears to be incorrect, incomplete, or inconsistent either on its face or on the basis of other facts known to the member. Further, a member should refer to the taxpayer’s returns for one or more prior years whenever feasible.”

When the prior return provided and the invoice for the accountant’s work for preparing that same return do not “match” or make sense and the client can provide no reasonable explanation, I feel like an additional inquiry to the accountant is appropriate.

SSTS says that I should not simply ignore the inconsistency. Yet it is clear that the only source of my concern over inconsistency is the accountant’s invoice and not any discrepancy in the tax work or filing. There seems to be no professional standard addressing this situation of communication with prior accountant outside of an attestation engagement under AR Section 400.

I’ve asked for peer guidance on this issue.