I would most definitely not answer the phone. They have a letter with your offer, and they are only calling to tell you NO. (And to confirm your current working phone number). If they were really interested in agreeing to the terms, then they would respect the terms which you laid out saying no phone communication and already have a letter in the mail to you.
One thing to look at is they finally had you contact them, with a PFD. You won't answer the phone, so any contact will ultimately come down to mail. While they are getting desperate, they may reconsider the offer you gave them.
On my first PFD letter, I had absolutely no contact with the CA, and never received anything from them. I had called them, big no no, to request something from them as I had sent three letters CMRRR requesting information on the account based on my CR, and an email. They sent the bill out to me, and I replied to that letter with a PFD. They used the phone number from their caller id when I had called to call me back. They actually left a voicemail. In it all sweet as honey and saying the call was regarding the arrangements to be made referencing the letters' date. I returned the voicemail and contrary to my good nature when dealing with morons, I was shocked into silence in listening to the cursing, lying, derogatory comments made by the woman who called me. I was shell shocked. Ended up hanging up the phone, sent a complaint to the FTC, and BBB, and haven't heard from the anyone since.
Bottom line, you made the effort to send a well written offer to them. If they choose to ignore your requests of mail communication only and try to get a feel for you, hoping intimidation will work, ignore it.
