Thanks for the heads up on this thread :-)
That’s really interesting. Thanks for reporting this important issue.
I was unable to reproduce this in our lab. s2Member® does NOT attempt to pass a phone number to Authorize.Net, and so long as you leave that field that you mentioned, unchecked, the Authorize.Net API does not require it either; which is good because many customers prefer not to provide a telephone number.

Since you’re reporting that Authorize.Net IS requiring you to provide that field (or, it actually appears to me that your installation is attempting to pass a phone number that is empty or invalid), I would contact Authorize.Net about this and ask them if your underlying merchant bank (i.e. Wells Fargo, or whoever you use), is forcing you to supply that information, regardless of your Authorize.Net configuration? Something like this?
Also, I know you’re working with some developers. You might check with them to be sure they’ve not modified s2Member in any way that would cause it to pass a phone number field, either via XML or via NVP? For instance, even if you attempt to pass an empty field (s2Member does NOT do this by default – it does not send a phone number field at all, by default), but if you attempted to pass an empty field, you might see this error.
Please let us know if problems persist :-)
Also, if problems continue, please submit a Dashboard login and we’ll run diagnostics for you.
Please do that privately: s2Member® » Private Contact Form