Several shipping carriers require a phone number.  Your sellers have checked the box in their preferences, that you must put in the phone number before you can pay.

If the sellers need your number for that particular carrier they can just request your information, and ebay will send it to them, it's just simpler to have you fill it in to start with.

Not all sellers have that option checked, because they know if they end up having to ship it with a carrier that requires a number they will simply pull the contact information and just fill it in that way. Other sellers (right or wrong) plug in their phone number, which doesn't help the carrier if they are calling to find the BUYER's house.

If a seller requires a telephone number that should be stated in the listing, not as a "surprise" in the Pay-Pal payment screen. I don't give out my phone number to anyone.

Unless I missed it in the listing and/or shipping info, it wasn't there.

Ebay buyers deserve to know ALL the seller's terms so that they can make an informed choice of whether or not to buy. Telephone numbers aren't needed for buyers and sellers to communicate, and are valuable to telemarketers. Unscrupulous sellers may make additional profit by selling phone numbers from their unsuspecting customers, and subject those customers to harassing telemarketing phone calls.

If buyers know upfront that a phone number is required, they can choose to purchase or not.

Oh the madness!!  When I ship a package via Fed Ex, Fed Ex requires a phone number for me to ship the item.  I have had customers who were happy that Fed Ex contacted them regarding specific instructions for leaving a package safely.  It is a nothing more than a courtesy.  If you don't want your phone number visible, at least default it to a series of 10 number 9s.