Red Flag #1 - A legitimate traditional publisher would never ask for a dime of money from an author. Instead, they will offer you what is known in the industry as an "Advance" on the estimated royalties. They will offer you a contract upfront stating how this advance will be paid out. As a responsible writer, you should hire a literary lawyer to go over this contract with a fine tooth comb. Remember too that you only get what you negotiate, and everything is totally negotiable.
Note: If you have to pay to get published, then your book is considered self-published. Be aware that in the publishing industry as it stands today, self-publishing is not acceptable work among most traditional publishers and many professional organizations do not recognize self-published authors. This doesn't mean it's not a well written work, but rather unproven.
Red Flag #2 - Any legitimate traditional or otherwise publisher will have there name out there, online and in other media venues. They are looking for work. If they are small, they still have professional references available and should be more than happy to share them with you. Always check their website and see what they have done in the past. Contact other authors who have worked with them to get their feedback.
Red Flag #3 - Why would a publisher not return a call to a client they have already contracted? Don't play the intimidation game that leaves you feeling you don't know what's going on, even if you really don't know. You should be asking what is this person hiding? A publisher is always busy as they certainly should be. There is no excuse for a publisher not to return your phone call within a reasonable time frame or to at least have an assistant contact you to let you know when you can expect a call back. The practice of common cutesy is simply good business.
Note: Here again, a responsible author will do their homework and research a potential publisher.
One last word of caution:
Never, never sign any contract with a publisher without going over it with a legal professional who can advise you and assist you in negotiations. It is well worth the money to protect your work and yourself.