With Yeastar S-Series and P-Series, all the telephony interfaces are modular and customizable, whether it’s FXS, FXO, ISDN BRI, GSM/3G, or E1/T1/PRI. S20 has 2 onboard module slots and S50/P550 supports 4. S100/S300/P560/P570 require the combination of Expansion Board and Telephony Modules to add telephony interfaces. S100 allows two Expansion Boards and S300 supports three. P560 can add one EX08 and P570 two.