Consider the type of hearing aid coupling options available to you (i.e., acoustic/mic coupling, inductive/telecoil coupling, direct audio input (DAI)). Certain accessories require a particular type of HA to telephone coupling method.
Consider the type of phone. Certain accessories will only work with very specific phone models and/or phone brands. (built-in communications jack for 2.5 mm plug or proprietary connector with an adapter)
Consider the cost of the accessory and its ease of use and maintenance.